I started my family tree on this site and now it's nowhere to be found. What happened to it?

There are two common scenarios that can cause a piece of the tree to "disappear": either you've inadvertently logged in with a different account, or another contributor detached a relationship.

If it's the latter, the good news is, FamilySearch's Family Tree is open-edit, which means that you can fix any errors you find, no matter who made them, and also that all changes are tracked, so you do not need to re-enter data: you can restore it.