Question about Find Familiar 2024

When my Find Familiar(Owl) HP Reach 0, it disappear forever or I can make it reappear with using Magic Action?

If it disappears due to dropping to 0 HP, you can bring it back by casting the spell again. The bit about bringing it back as a Magic action only applies if you have previously dismissed it temporarily, not if it has actually died.

Druids have a level 2 feature that allows them to cast Find Familiar as a Magic action, and Warlocks have an Eldritch Invocation option called "Pact of the Chain" that allows the same thing. But if you don't have a special feature like this, you need to cast the spell normally, which takes 60 minutes if you're using a spell slot or 70 minutes if you're casting it as a ritual.

You have to cast the spell again for it to reappear and you can choose the form again. 

I dunno if the forum is still viable to talk about, but asking for a friend of mine, can a familiar dismiss itself after being summoned?

Quote from KrowKatz_24 >>

I dunno if the forum is still viable to talk about, but asking for a friend of mine, can a familiar dismiss itself after being summoned?

The spell description doesn't give familiars that ability, so technically that's not a thing they can do according to the rules.

In practice, many DMs (including me) like to give player characters' familiars a lot of personality and sentience, and that often comes with more autonomy and independence than are strictly called for in the rules, so it's definitely something to check with the DM about. 

I'm not sure what "the forum is still viable to talk about" means here.

Not RAW. Talk to your DM.

Quote from KrowKatz_24 >>

I dunno if the forum is still viable to talk about, but asking for a friend of mine, can a familiar dismiss itself after being summoned?

 The ultimate abandonment - left by your familiar 10 minutes after it meets you.