Under window->settings/preferences->preferences you can find a settings->undo page. Have you checked those settings?
If caps lock is on ctrl+z will not work, you will have to turn caps lock off or use ctrl+shift+z. It might sound stupid but it had me puzzeled for 30 seconds.
Under window->settings/preferences->hotkeys you can change the hotkey for edit->undo. You could check that setting.
Other than that I don't have any suggestions, and you probably knew these already.
Otherwise there might be some file somewhere containing the settings, you could try to move that file to a place maya wouldn't find it, but I don't know what file that is. There might also be some command for resetting all the settings in maya, but I haven't found that either.
Hopefully you don't have to reinstall.
In the begining God created a whole lot of rigid bodies, and set some motion paths. He also set the lighting.