Undo Redo Disabled

I have an excel that someone sent me and as soon as I open it I cannot undo or redo things any more. I cannot undo anything in any open Excel workbook anymore. How do I enable Undo again?

Added: There is a macro in it. Is it possible that the macro disables it?

yep depends on the macro but once it runs it probably clears the undo stack

Check out your worksheet and workbook change events. Make sure there isn't code somewhere in any of them in some unrestricted manner; i.e., only applied to certain rows and/or columns. If a change event runs for any change made anywhere, it will disable the undo and redo abilities.