I was minutes a way from reinstalling Ubuntu on my home machine because it wouldn't let me log in all of a sudden. I saw the GNOME login screen which looked different than normal but all my user accounts were still in the list. I tried every account and their passwords but none of them would let me in. The login screen would go away like it wanted to log in but then just bring me back to my login screen, like it was caught in a loop. I never got the incorrect password prompt so I figured maybe I had to reset the password.
My grub menu screen wouldn't come up during boot so I instead booted from the Ubuntu installation media and followed this tutorial which worked great except when I rebooted I had the same problem. Ugh. I was still able to log into the terminal with CTRL+ALT+F1 and could see that all my data was there. Strange. Maybe a problem with the nvidia drivers or ubuntu desktop? I tried reinstalling all of that, same exact problem.
Then it dawned on me that maybe the hard drive was full? Not likely, but possible. A df -h from the terminal showed me that there was 0% available on my HD. I removed a couple large folders and viola, back in business on a reboot.