If you recently did the Ubuntu Feisty Fawn maintenance upgrade of the kernel from 2.6.20-15 to 2.6.20-16, your system probably doesn't work: in previous versions of the kernel, all your SATA disk drives were considered to be SCSI drives, and so the partitions were mapped to /dev/sda1, /dev/sdb2, etc. 20-16 breaks this, considers the disks to be IDE drives, and tries to map them to /dev/hda1, /dev/hdb17, and so on.
Fortunately, there's a quick, if temporary, fix: when you boot up your computer, hit the ESC key as soon as you see the message about “grub”. Then use the arrow keys to select the 2.6.20-15-generic kernel. Proceed with boot and all will be well.
You can edit the /boot/grub/menu.1st file to make the 20-15 kernel the default. I'd advise that if you have non-geeks using the computer.
Presumably a fix will be along in a few days, version 20-17 of the kernel. When this is installed you may have to edit your menu.1st file to make the new kernel the default.
Sigh. Things like this happened occasionally with Fedora as well. I hope there aren't too many newbies scared away because they think Ubuntu broke their computers.