Do you have a save point prior to the update?
Another thing to try is to put in a bootable disk, restart the computer and see if the computer will start from the disk rather than the hard drive. You'll have to have the boot order in the bios set to the optical drive first, then the hard drive. This will insure that the optical drive is still functioning and that it simply hasn't failed. Sometimes what we think is a software issue is just a coincidence in timing with something we've done on the computer, and it is really a hardware issue. Trying a bootable disk will rule this out.
An operating system disk will work as a bootable disk. You can also get one online, like http://www.nu2.nu/pebuilder/
While you're in the bios, make sure that the optical drive shows up in there as well. If it doesn't, you can be certain there has been some sort of hardware failure.