Graham has told you how to restore it. I will explain how you lost it:

1. You wanted to insert an em dash. The keyboard shortcut for that is
Alt+Ctrl+Num- (that is, the minus key on the numeric keypad).

2. Instead, you pressed Alt+Ctrl+Hyphen, which is the shortcut for
ToolsCustomizeRemoveMenuShortcut.

3. You didn't notice that the mouse pointer had changed to a big minus sign,
but you didn't get an em dash and thought nothing had happened.

4. So you thought you'd insert the em dash from the Symbol dialog and
clicked on Symbol on the Insert menu.

5. What you didn't realize is that the function of
ToolsCustomizeRemoveMenuShortcut is to remove any menu item you click on.
This is handy for removing unneeded files from the Work menu or deleted
files from the MRU list at the bottom of the file menu (or any other
unwanted menu item) but disastrous if you're trying to actually *use* a menu
command.

If this ever happens to you again, be aware that you can dismiss the command
with ESC.

I solved the problem according to your instructions. Thank you so much.
Alberto