Hi,
Shift+F5 should do this. Alt+Ctrl+Z should also do it. They do on my system.
If you want to attempt to fix this issue, perform the following steps:
1. Click the Office button.
2. Click Word Options.
3. Select Customize.
4. Next to Keyboard shortcuts, click Customize.
5. In the Catergory list, select All Commands (near the end of the list).
6. In the Commands list, select GoBack.
At this point, Shift+F5 and Alt+Ctrl+Z should appear in the list of keys
assigned to GoBack. If Shift+F5 does not appear in this list, click inside
the Press new shortcut key box, press Shift+F5, and click Apply.
If Shift+F5 still does not work, press Alt+F8 and look for "GoBack" in your
list of macros. If you have a macro named GoBack, click Edit, and change the
name of that macro.