Could be a policy thats disabling the menu bar? I've seen something similar where task manager is grayed out (caused by spyware) so that you can't end the task.

To check go start, run, then type gpedit.msc I'm not to sure where this policy would be but i'd take a look at user configuration -> administrative templates -> Windows Components -> Windows explorer first and check if anything in there is disabled. You could also have a better look around and see if anything else is disabled that sounds like it could be the issue.

Failing that what I'd do next is either create a restore point (today), then restore back to a date it worked and see if it does still work, if not you can roll back to today. You could also create a new user account on your PC and check if the new user account has the same issue, if not then it is account related so you could copy your docs/settings etc into the new profile and delete the old.

Finally it might be worth running some antivirus/spyware programs on your machine to check if its clean incase it is a similar issue to what i have seen with the taskbar.