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.
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.
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