can't log in, until

[Aelita]

OK, I fell asleep with my computer on and playing music, when I woke up and tried to unlock it, it wouldn't let me log in, I know I entered the right password, I tried switching user (I am only user) and when I did that, It let me log in with the same password.

What I'm asking is why this happened? Any ideas? And in case it happens again and I can't fix it like I did this time, how to fix it?