Depending on the problem that may be causing the missing menu bar, this could trigger Word to do some repairs to itself. If it doesn't, try starting Word from the command line with the /a switch. Hopefully, this will display your menu bar. Word displays a dialog box asking if you want to reset changes to the menu bar.
If you run Windows at a high resolution, undock the menu bar and move it to the far right of the screen, and then change to a lower resolution in Windows, it is possible that the menu bar won't be visible because it is now outside the area of the screen you can see. First, because the menu bar is treated pretty much like a toolbar, it is possible to undock it and move it anywhere desired on the screen.
Theoretically, the menu bar should not be "losable," so when such an event does occur, it is a good sign that something is wrong. Dave reported a situation in which one of his Word users had somehow completely lost the menu bar.