A few days ago I turned on my pc and couldn't login with my username tu Ubuntu 13.10. I don't know the reason, but I tried all what I found in internet and didn't work. Until now, I have been using the same version of Ubuntu for 2 months without any trouble. But now, when I login I go back to the login screen. As a guest I could login perfectly.
I think this may be a bug. This happen to me days ago. What i had to do was create a new administrator account from the guess account. Yes! You can do it from the Guess account. Use the same password you created Ubuntu in. Then get all your files from this new account and then delete the old account. A bet when you typing in your password it tried to log in and sends you back to log in for password. I'm still trying to find out what caused this to happen. I'll update if i get to something.
I would recommend you do the steps I take on solving my login problem, here, on my answer to other question: Solve login problem