Well this is a tickler. After I performed an update on a CentOS system, the top menu pulled a Houdini on me. The panel was totally empty and non-responsive, save for the time.
This is a quick and easy fix, so don't panic. Just force the gnome panel to reload.
0. Login as root or sudo.
You should find the Terminal via right-click on the desktop. Thankfully.
1. Reload the gnome panel (forcefully)
2. That's it!
The menu items should be return. If that didn't work, you might need further help.