Gnome's shutdown menu disappeared

Solution 1:

You need to add the "Indicator Applet Session" applet. It's in the "Add to Panel" list.

The "Shutdown" applet gives you a popup dialog with Shut Down, Restart, Suspend and Hibernate options.

The "Indicator Applet Session" applet gives you the drop-down menu with Log Out, Suspend, Hibernate, Restart and Shut Down options, as well as Lock Screen, Guest Session, Switch from... and other fast-user-switching options.

Solution 2:

Right click the toolbar on a spot with no button and go to add, then add the button back. Drag the button to where you want it, and you may be able to lock it in place so this doesn't happen again.