If you have no other menu customizations, the easiest way is to choose
Tools/Customize/Customize Toolbars/Menus...
In the Toolbars pane, select Menu Bar (so that it's highlighted, not
just the checkbar checked) and click Reset.
If you have customizations you want to save, in the Customize
Toolbars/Menus... dialog's Commands pane, select Categories: All
Commands, then drag InsertSymbol from the Commands listbox to the menu
toolbar/Insert popup control.