Two possibilities occur to me (neither may be the cause of the problem, but they might be worth investigating)
1. Is nomodeset being fed to the kernel? If may have been if you were using the vesa driver. If so, it's time to delete it from grub.cfg (via editing the proper config files in /etc and running grub-update, of course)
2. Do you have an xorg.conf file? If so, is it properly configured for the nvidia driver?