Your previous update probably didn't update the grub. If you got a new kernel as part of the update, grub may not be able to find something to boot, especially if the old kernel got removed as part of the update. (I don't know if Ubuntu does this; I know RH-family don't.)
What I usually do when Grub gets unhappy (assuming my grub.conf is set up correctly and the kernel is in the right place):
- boot from a rescue CD of some sort. Ideally one with the same version of grub that you have installed.
- re-setup grub from the rescue environment:
> device (hd0) /dev/sda
> root (hd0,0)
> setup (hd0)
(blah blah blah)
Running "install /boot/grub/stage1 (hd0) (hd0)1+16 p (hd0,0)/boot/grub/stage2
Then sync and reboot. The partitions don't have to be mounted for this to work.
Things to know if you've never done this before:
- Your disk is always (hd0), even if it is sdb, hdc, whatever.
- The "root" partition is the partition with the kernel in it: /boot if you have one, / if you don't. The second number is the partition number minus one. So if you have /boot in partition one, it is (hd0,0); if it is in partition two, it is (hd0,1), etc.
It should complain if it can't find what it is looking for and shouldn't be destructively harmful (it shouldn't hose you any harder than you are hosed now). Usually that means that either your disk can't be found (you can test that by mounting it) or you've specified the wrong partition as the "root" volume.