"when I try and login (debian, Gnome or fluxbox) through GDM - GDM returns me to same login screen"
You can start diagnosing this problem by seeing if you can log in to the command line. When you are sitting at the GDM screen use the three finger command <Ctrl> <Alt> <F3> to see if you can get a command line login prompt. If you can then try logging in as root and all of your user names. You can logout with the logout command. You can shutdown with:
shutdown -h now
Be prepared. Create a LifeBoat CD.