Some people have had SharePoint stop working after the installation of Service Pack One. This is mainly due to the upgrade process not completing. You can force an upgrade by running the following command from a command prompt:
psconfig -cmd upgrade -force
This will force the upgrade process to run again and should fix the problem for you.
# Added 11 Jan 2008
You might want to also try this:
Click Start, click Run, type cmd in the Open box, and then click OK.
At the command prompt, type the following lines, and then press ENTER after each line:
cd /d %commonprogramfiles%\Microsoft Shared\Web Server Extensions\60\Bin
stsadm -o upgrade -forceupgrade
Let me know how it goes.....