I am an amateur DM with a group playing a homebrew 5e. I have 5 players and we are all very good friends, but I was the only one willing to DM. We started with the Mines of Phandelver and slowly switched to my own story, but we are only on the first leg of my part of the campaign and I’ve backed myself into a corner by going too big too fast.

I set up this fairly intricate town, but made the mistake of not having enough random encouters. Then I made the mistake of expecting my players to follow along. I had created this battle arena competition, but one of my players refused to join the competition. The next few sessions had me panicing as this players was bored and unengaged, frequently leaving the table to play on their switch or fiddle around in their kitchen. In order to engage them, I changed my plan for the city, making it much more intricate and dramatic, then rushed the plot  so they could feel involved.

Now, I’ve somehow got a huge battle set up for the next session and I don’t know what to do. How do I use mechanics for a huge battle? Also, I basically had the players abandon my battle arena plot, which left some of them feeling unfulfilled and is also a problem since the prize is one of the main plot driving items. How do I get them to the prize? How can O resolve this battle? I can go knto more detail if I need to, but right now I’m at a loss for how to fix it. Any help is welcome. 

You need them to get an item, and there is a battle of a scale you can't handle coming up? Firstly, a giant battle of hundreds need not actually happen with dice. You can just describe that it is happening in the background, but then keep the corner of the fight where the players are located at a more manageable scale. Maybe they are facing off against some smaller scout group, or some elite forces and a leader, while everyone else is just background noise. 

Now, you need them to get the item which is the prize, but not all of the party is invested in winning the competition? Time to bring in an agent of some kind. Some shadowy NPC full of exposition approaches the least invested of the party members. "We heard some of your associates are involved in the competition. We need your help to keep that item out of the wrong hands. Here's the plan, you use the cover of the battle to swipe the prize (or use this assistance to win the competition). In return we offer you [insert exactly the sort of thing this party would desire]."

On a smaller scale around maybe pushing yourself too far, there could be a potential player problem to look into. What were the reasons the player was so against the "arena arc" that you had come up with that they refused to be involved with it along with the rest of the group? It seems that it was particularly obtuse, especially that they then chose to remove themselves from the game so... obnoxiously through want of a better word. There should have been stuff that could have kept them more involved if the party discussed what they wanted to do and plan around it (masquerading as the team manager, suspicions that a rival team are "enhanced" unnaturally and investigating that or trying to socialise a bit with the people for gossip and odd jobs) so that the player chose to abandon the game to potter about elsewhere is a bit worrying to me as a DM.  That you called off the arc early to try and reintroduce the wanderer and upset a couple of the other players is a shame, although I understand why you did it I'd like to know why they chose to have nothing to do with the game at all. I've assumed that there was some party discussion around this and you didn't just say "You're enlisted to take part in this tournament whether you like it or not" ;)

BigKahuna has a lot of good advice regarding the Kick Down the Door approach and published adventures while picking things up. In fact as a top of the head example the scenario described is quite a good link. The attack comes early, catching the town and party off guard, there's a dragon involved and you've got a fairly natural meld into "Hoard of the Dragon Queen" just by changing the location to your current town.

A lot of DMs can get quite deep quicker than anticipated and find themselves overwhelmed and without another experienced DM in the group to try and assist you it's even easier for you to find yourself pushing things too fast. I'll be honest though it seems that a fair amount of your problem has stemmed from a "panic switch" being used which may not have been anything at your end. Maybe you could have had a bigger "urban encounter" list to work off with regards to random events or had a couple of bigger town arcs as alternatives to the arena event ready to go, ultimately leading to the same reward but exactly how they gained it would depend on the adventure they went on.  Winning the tournament, rescuing a nobles daughter from a gnoll raiding party or uncovering and preventing a plot to assassinate a major figure within the town for example. For now though I would again recommend the big story shift while you pick up some more experience.

This is all really great advice. Thank you! I think I have some ideas now to get myself out of this pickle.

I just ran a big battle with my large group and I had a mix of smaller missions for them that furthered the battle and helped out their side, like run here and make sure these supplies get to this front or go over here and drive the front forward.

And then when they were in a giant clash I would simply describe what was happening all around them ( it was a massive battle between goatmen and dragon born) and let them know there was no shortage of targets to choose from and maybe give a specific instance or two like, you see a dragon born biting the face off a goatman as another runs up to hit the dragon born from behind and basically let them tell me what their action was on the field of battle.

It worked great, they all had fun for hours and were stoked for the next session.