Pressing ctrl+m hides the menu bar. If this is accidentally done, (which happened to me,) it's a complete mystery for how to get it back. There's no obvious hint, and it was unclear what I should google for. This seems to be a fairly common problem:

http://ubuntuforums.org/showthread.php?t=2227964
https://forum.xfce.org/viewtopic.php?id=8880
http://askubuntu.com/questions/555114/xubuntu-thunar-menu-bar-disabled-now-lost-forever
http://ubuntuforums.org/showthread.php?t=2281759

A simple fix would be to add a confirmation dialog when it's first used. This can explain how to get the menu back later on, and prevent accidental hiding.