Cannot login after update to Ubuntu 14.04

After I updated to Ubuntu 14.04, I am not able to login to Ubuntu.

Whenever I enter my password, my screen flashes for a couple of seconds and goes back to the login screen. I have tried over and over again but the same thing happens.

I can't login through the guest session either - whenever I click login, the screen flashes and goes back to the login screen again.

What do I do? I'm really confused and stuck and I need to fix this problem REALLY soon!

Thanks!