OS X Snow Leopard fails to boot after upgrade to 10.6.1

"You could try booting from the original CD and reinstall Leopard, hopefully without losing data.

If you have a firewire port on the machine AND someone else with a firewire capable computer (and a cable), you could put the computer into target disk mode to copy all your data off and start with a fresh clean install of snow leopard.

You should think about using Time Machine to keep your documents and data safe, as well as a third party backup solution like Mozy (2 GB free) or SuperDuper to make a bootable copy on an external drive."

"Try using the ""Repair Permissions"" option of Disk Utility. It's in Disk utility, after you click on your start up disk, under the first aid tab, down on the left.

Here is an Apple support article that explains what it does.

In any case try the backup that brandstaetter suggested first."