First off wait a bit, you may come back.
Second off, were you actually ranked? Were you checking from a private browser and ideally a different IP? Did you get any traffic? Could've showed you at the top when you weren't actually there.
Third, if your video was ghosted from the YT SERPs... it's usually best to just try again with a new video and a new account. If you're whitehat and dead set on using this video and this account, you could try changing the title/description around too -- optimize it.
Perhaps your purchased metrics dropped too, which led to the SERP drop, though I haven't messed with purchased metrics in a while.