...or boot in single user mode from Grub (i don't know if you have already tried this as you said in the first post, but it's really strange it's not working.)
At login screen, try Ctrl+Alt+F1...F6 to switch to one of the textual consoles, and try to login and check from there if your credentials are OK.
Otherwise follow ostendali's way (if you need, follow an how-to about chrooting.)
Bye.