Ubuntu is not booting properly for me anymore. The grub menu comes up, and whether I choose a linux recovery mode or the normal one I get a black screen after a few seconds. There is a brief message about vga=791 being deprecated, but I am not able to read it fully because the black screen covers it up almost immediately. I have googled for hours for solutions, and most people seem able to solve similar problems by editing in grub and adding nomodeset into the line starting with linux /boot... but this solution is not working for me, I still don't get any GUI. Sometimes I am able to get the dmesg rolling past, I think it was when I removed quiet splash from that line, but still no GUI - the computer seems to be on and working because it responds to a ctrl-alt-del and reboots.

I have tried with 3 different graphics cards (2 nVidia and 1 ATI) and swapping them doesn't seem to change the behaviour at all. What else can I try?