When this happens, can you press Ctrl+Alt+F1/F2/etc and switch to another terminal?
Can you press Ctrl+Alt+Backspace and restart X? Have you checked /var/log/XFree86.log.0? (might be several) >From where did you install the nVidia drivers? When did the problem start happening? Have you tried backing up your X config and using either dpkg-reconfigure or apt-get --reinstall on X? Have you considered upgrading to a newer kernel?
