My party all walked into a mirror of Life trapping. The first player was the warlock, who is the only one knowledgeable with the arcane so he investigated and failed his saving throw. I described it as him feeling compelled to walk into the mirror and vanishing on the other side. The barbarian, thinking the warlock knows what he is doing followed. I ruled it as he is intentionally tryin to go into the mirror he automatically fails the saving throw, like an ally's polymorph. The rest of the party decided to follow suit, thinking that either that was the right way to go or the first 2 would need help getting out. 

Rules as written, My party just TPK'd themselves. They have no way out. Maybe I should have made them all roll saves. Maybe I should have described it as more forceful, It was a charisma save so I thought a charm type effect. The worst part is the Warlock has the command word he just doesn't know it. Maybe the player does and isn't metagaming. They haven't identified the mirror so it doesn't have to be the Mirror of Life Trapping exactly. There could be some sort of loophole in the prison dimension. 

Any suggestion for how to alter the Mirror so the party can escape? Some sort of mirrror dimension encounter. Hopefully something more interesting than fighting their own reflections, I've already used that. My group likes Puzzles anyone know of a mirror themed puzzle?

My other option is the BBEG returns to collect his mirror and see who fell into their trap. Then the party becomes prisoners of the BBEG and need to escape.

I know it's not my job to keep the party alive but this is a really unsatisfying way to end and there are a lot of open ends here.

P.s.

An extra complication is one of my players was away. Normally I just have missing players reappear beside the party no matter what situation they are in. It kinda feels lowsy to tell the player "you're now trapped forever cuz dumb decision the rest made" But if I make up a narrative reason for the absense then I need to have that player solo explore the dungeon until they find the mirror and hope they don't get sucked in too. Or I could start next session with them observing the rest of the party walk into the mirror and ask if they follow

Personally, I would consider this an opportunity to run a Through the Looking Glass themed adventure.

Ignoring the missing player for the moment, the obvious question is: Whose mirror is it? If it was somebody the party just murdered, well, now there's somebody else as well, who wasn't there at the time. They have a mirror with brand new prisoners in it. What will they do with them? It can't just be "let them rot", because eventually the mirror fills up and becomes useless. They're going to want to pull their prisoners out and use them for something.

Or you could say "this is an atypical mirror of life trapping", and run an adventure trapped in the mirror world. until they escape. Time to be weird and dreamlike.

That said, the missing player is also your out. Start your next session describing to them what happened. Either they go in, in which case see above. Or they do something else, which is likely to end with "break it". In which case, problem solved. (New problem: whatever else is in the mirror.)

You could make the interior of the mirror a sort of labyrinth that they must figure out to escape instead of just being infinite, empty cells.  They could potentially run into people or creatures that have been trapped in there for centuries.  That makes it essentially a dungeon with the exit being the prize at the end. 

Maybe to get out, there's a replica of the mirror in there, and there's a crystal that must be set into it that becomes the portal out, but to acquire the crystal the party must face its guardian (ex: the crystal could be one of the eyes of a dragon that's a construct of the mirror).

The mirror doesn't kill anyone. It is a prison. Everything trapped in the mirror is released when the mirror is destroyed.

Where is this mirror? Who owns the mirror? If it was left active then either the original owner is using it as a defence or the original owner activated it and later died so they couldn't turn it off.

Options-

1) Original owner (or someone with the command word) comes back and decides to do whatever they want with the new prisoners - you can take the plot whichever way you like with that. (A kindly caretaker might even just release anyone trapped .. if you want an easy out).

2) The original owner isn't available, the mirror has been left forgotten in the corner of a dungeon. Eventually another traveller may come around. When the 12 cells fill up, the next person is trapped but the first is released. Eventually, the PCs will start to be released - though it could be a long time. You could use this to skip the game ahead a year or more, even a century if you feel like it depending on how often the location with the mirror might be visited.

3) If the mirror is destroyed, all the creatures are released. Again depending on where the mirror is located, you could have someone come by and destroy it because it is magical. Or perhaps it is destroyed by accident or as a result of a natural phenomenon. An earthquake or partial roof collapse, again depending on where the mirror is, could see it destroyed and all the creatures released.

There are many fun ways you can take this one since it is not a TPK. The characters will eventually get released from the mirror one way or another - it is up to you how that happens and how long it takes. You could allow centuries to pass - leaving the characters in a dark future, perhaps as one of the few remaining forces for good and they need to find out what happened and then try to go home to change something so that the dark future never happens. :)

The mirror was given as a "gift" to the owner of a Tavern. The Tavern owner was a member of the rebels fighting against the evil local lord. It was Intended the trap the owner and removing the threat but instead it trapped the owner's kids. She went mad trying to figure out how to free them, she believed smashing the mirror would either kill those inside it or trap them permenantly. She eventually goes mad and kills evreyone in the Tavern thinking they must all be spies trying to trap her. Leaving the tavern haunted and abandonded. The party has arrived looking for a smuggler's key the Tavern keeper once held.

I could have the Evil Lord's henchmen return for their trap. Possibly breaking it while returning it to the evil lord. Or have the Evil lord empty out the mirror into his personal prison so he acn use it again. The hotel is in ruins so the roof collapsing isn't totally unbeleivable. The setting is already a "What if the bad guys won" type setting so a jump into the future feels like overkill.

Having the mirror broken by random occurrence (NPC or Natural) feels a little too easy on the party. Equally unsatisfying to the "You're now trapped forever" ending. After mining the internet for Ideas I think I've got Either the BBEG returns to collect his mirror and this is a good way to properly introduce the party to the BBEG face to face. Or it is a faulty Mirror, housing interdimensional demons who have weakened the walls of the prison.

You could go for the Plane of MIrrors (link to wiki here: https://triplecrit.fandom.com/wiki/Plane_of_Mirrors), have a few sessions of them exploring the plane and trying to find a way out, maybe even have them fight their mirror opposites. You can then use this to move the plot onto another location as they exit the plane of mirrors and can appear pretty much anywhere there is a reflective surface.

Sounds like a bit of a Diastereomer and a bit of a puzzle to find in a load of Enantiomers but possibly a fun one
If you decide on the puzzle option of escape then I'd tend to having the absent player having joined the group 

You could have them meet a somewhat mad prisoner who needs their help to escape. He has a "foolproof plan" but he needs the party to help him get two items for him from two of the other, nastier, inmates of the prison.

These two items both happen to be bags of holding. His grand plan is to put one inside the other, then escape to the Astral plane in the resulting explosion.

It'll probably derail the campaign, but as they're TPK anyway, it could be a good little excursion before they try to get back. Or he could have the ability to cast a teleportation spell and get them back to the inn, after the yescape the mirror.