Office challenge: Why doesn't [Ctrl]+z work anymore?

This week, test your general Office skills by discovering why a user's [Ctrl]+z shortcut no longer cancels the last action.

A user calls in a panic-she uses [Ctrl]+z all the time and suddenly, it's not working! Instead of cancelling her last action, nothing happens. What terrible gremlin has been at work in this user's system and more importantly, can you restore [Ctrl]+z's default behavior?