Can you login via console? Press CTRL+ALT+F1 and you should get a text login..fill in your username, hit Enter, fill in your password (it doesn't display anything on screen while you're typing, be careful) and hit Enter again and see if you get logged in. This makes sure you type the username/password correctly, and that your account works. After that you can view GDM's (Gnome Display Manager, it's the thing that handles graphical login, that you were talking about) logfiles or X's logfiles under /var/log to see if there are any curious error messages:
The above would show you Xorg's logfile, for example (press 'q' to quit, arrows to scroll).
Shows contents of /var/log (logfiles and possibly some directories with more logfiles in them);
Moves the current working directory into /var/log and so on..with these you should be able to read the logs to see if there's something going on. The newest information is at the end of the logs.
Another thing you can try is to create a new temporary user (whose settings are "clean") and see if you can log in with it -- if you can, then you've got some settings trouble with your normal user that you can get rid of by removing some settings files (settings are reverted to the original values, which you can then re-adjust again); or if you still can't login, it's something with gdm itself (then you could try to reinstall it, for example). To create a new user,
sudo mkdir /home/tempuser
sudo useradd -d /home/tempuser tempuser
sudo passwd tempuser
After the last command give this new 'tempuser' some password so you can login with it, hit CTRL+ALT+F7 to get back to graphical side, and try to login with user 'tempuser' and it's password. If it doesn't work, try CTRL+ALT+BACKSPACE which restarts X (and GDM), and re-try; if it still doesn't work (or if it does), you can remove this temporary user; get back to the text console with CTRL+ALT+F1, and
sudo rm -rf /home/tempuser
sudo userdel tempuser
And then CTRL+ALT+F7 to get back to X. That's a start at least..