Well... Say a star falls a long distance... and its light goes out. And maybe it stays in its own darkness for too long. Turns away from its proper place. And no matter how much people... or other stars try to reach out to it, it just... won't listen.

I think that's the point when it can't ever return. When it's fallen far enough that it thinks it doesn't ever want to.

Or if it wronged other stars on its way down. I could see it not being able to return then, too. If the other stars won't let it.