NM,

I see two possible things, both of which are very unlikely, in my opinion:
1. Registry settings, like the ones mentioned in http://www.jacobsen.no/anders/blog/archives/2003/12/02/greyed_out_ms_outlook_options.html
2. You somehow lost the right access rights to your Outlook database (that's by default c:\windows\outlook.pst, I think, but you can easily check so).

If #1 is the case, a system restore would do the trick.
If it's #2 you'll have to check the file and the access rights, and possibly restore a previous version from your backup.

Better wait for brighter ideas.

Kees