Discussion on:

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

Wast the document saved? If so then ctrl-z doesn't undo anything until something s done after the save. In other words: save clears out the undo list.

My comment was greyed out but not saved... meh.

my guesses:
1) keyboard layout was changed from standard US to DE, FR, RU or whatever else, so a Ctrl+Z is no longer a Ctrl+Z;
2) a newly-installed app has hijacked shortcuts (Intel video drivers and some Polish national diacritics, anyone?)
3) a retrieve has been made with Excel add-in for Essbase (granted, this is rather exotic, but does happen... and hurts when it does).

Perhaps the user (or creator of the spreadsheet) has, unwittingly, assigned a macro to the shortcut?

If it worked yesterday and doesn't work today, try a reboot.

Sure, the reason for Ctrl+Z not performing the undo function may be that a macro was recorded using Ctrl+Z as its keyboard shortcut. Now, the question is, how can you remove the keyboard shortcut from the macro? Another question is, how can you add a keyboard shortcut after you've already written a macro?
If anyone can answer that, I'll give you a hundred points.

You can add a keyboard short cut by accessing the Keyboard shortcuts dialog box under the developer tab and highlight the macro. In the dialog box, under Categories, select the named Macro. Then underneath in the "Press new Keyboard shortcut:" box press the keys you want the shortcut key to be, such as Shift+Ctrl+Z, then click Assign. Check to make sure you haven't already assigned the shortcut key to another function by looking through the "Current keys:" list. Also, you can assign the keyboar shortcut to other templates besides Normal, if you want to customize another template like a letter.