The same thing happened to me in Ubuntu 9.10 - by the way.
I have a new installation on a new hard drive and everything worked ok (Mint 8 - all upgrades ok, Nvidia GeForce 7600)
After I tried to install the Nvidia driver I can not boot very far. I get past the initial text 'grub....' and up to the green 'm' - then nothing; i.e I do not get up to the user/ login screen. The screen goes black and all I can do is power the whole PC off. I am a beginner and can only operate the terminal commands if given a recipe; i.e cut and paste.
Currently I operate via the live CD - but can not get to my programs and data easily, especially my mail, etc.
How can I somehow get rid of this Nvidia driver (and rather carry on working in a plain display mode) but get back to starting up all the way into my nicely working system? Unless, of course, there is a solution and I could use the Nvidia driver.
Thanks - Harry