I had this "Input Not Supported" problem today after a Xubuntu update (12.04).
To start off, if you can see the Grub menu (with choice of kernel/OS, etc), then your Grub graphic mode is working okay. Otherwise, adjust:
in /etc/default/grub to suit your video hardware, run grub install, then reboot.
After Linux boot starts (we're done with Grub now), this line in /etc/default/grub is what sets the video res.:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash ... nomodeset video blah 1280x760x=-blah blah blah"
After wasting an hour of my life I'll never get back, trying different settings of this one line, I said screw this and erased the whole thing:
When I booted with that, voila!, it all booted very nicely as if nothing had ever gone wrong.
You see, I had a bunch of crap in there from previous attempts last year to get Plymouth splash screen to work. Never did work, prob. due to my pesky Nvidia adapter.
So now I just have "quiet splash" back in there -- but nothing else. The "quiet" suppresses the boot messages and "splash" enables the default splash screen -- in my case that's not much bec. Nvidia and Plymouth don't seem to like each other. Works nice on shutdown however!
So this week's Xubuntu update broke the boot for me. But I think it means that the nice Xubuntu/Ubuntu folks fixed things -- it was my complex boot parameters that screwed things up.
BTW, if you're ever in this situation again, once you can see the Grub menu, you can actually try things without using the shell/editor/grub-install. Just go into Grub's edit mode -- see the bottom of the Grub menu screen -- letter e. Modify settings in the "linux" line there and your new settings will be used to boot (but not saved anywhere for next time).
Here's some good info about Grub settings: