This question already has an answer here:
I upgraded to 11.04 using update-manager -d. When I try to boot, I just get a purple blank screen, with no ubuntu logo/dots showing. It never gets to the gdm login screen (I left it for 20min) and I can't get to a tty by pressing Ctrl+Alt+F1. I don't see a grub menu/count down screen either.
My computer is a Dell Vostro 1500 with a Nvidia Geforce. I had the proprietary nvidia driver installed before upgrading.
I've boot with a live cd, from where I am writing this, and I can mount the disk, but I don't know where to start looking for the issue.