In recent versions of MacOS, there is an option in System Settings >
Desktop & Dock > Menu Bar to "Automatically hide and show the menu bar"
which you may need to change.
Possibly they are accidentally going into "full screen app" mode (try
pressing Command-Control-F to toggle full screen on and off). Maybe
check which gestures the trackpad is able to use - there might be one
there for hiding teh menu bar / dock that they are doing by accident.
(Whenever I set up a new Mac, I turn off all the trackpad / Magic Mouse
gestures, among many other annoying "features", especially if the
computer is being owned by a novice user. The accidental gesture is one
of the biggest causes of "my computer is doing something weird"
problems.)
If that doesn't help, then it's likely a bug. I have noticed that
happen very occasionally for decades (why back to System 6, so long
before MacOS X), sometimes due to an app crashing, especially a game
that hides the menubar. A reboot is probably the easiest way. Other
possible quicker fix might be to Option-click on the Finder in the Dock
and choose "Relaunch" (you may need to be in another app to see the
Dock first though).