My Youtube app won appear anymore if I click home. What's wrong?
It depends on where you are from. What you're seeing is the PIP mode of YouTube.
For most parts of the world, it is only included with YouTube Red. So, maybe you had a YouTube Red/Pro subscription or Trial which has expired. 

YouTube also enabled the PIP for free to all users in the US. That is, even those who don't have a paying plan. So, do you happen to be from the US? Plus, they could be testing the free PIP support in in other regions too.