This bug has been a problem for a while now when running debian testing - if you do a dist-upgrade, chances are your prorietary drivers get borked.
Symptoms include gdm not starting i.e. no Gnome login window. Instead you get dumped into the terminal.
There are two fixes - one quick and dirty one and one which is more long term.
1. Quick and dirty
If you want Gnome up and running without necessarily using graphics acceleration, simply log in in your terminal and rename your xorg.conf.
sudo mv xorg.conf xorg.conf.old
This will allow you to use the graphics card on your motherboard again. It won't help with a PCI card much.
2. A real fix
SMXI. It may seems scary, but is fairly straightforward. Log in (presumably in the terminal)
cd /usr/local/bin && wget -Nc smxi.org/smxi.zip && unzip smxi.zip && smxi
The sudo su logs you in as root. You could also sudo each command individually above.
Follow the instructions and let the smxi script install your graphics drivers.