installation - after 12.04 upgrade: can't log in although password is correct

I upgraded from 10.10 to 12.04, and now can't log in any more. The password is correct- when I type in a wrong one, I am told it's incorrect, while the correct one is seemingly accepted, but then just takes me back to the login page.
I can login as guest, but I can't do anything with the shell as it, too, doesn't accept my password.