I have used Exel for many years and never experienced this problem before.
One day I opened a file and it was okay; I could undo and redo with no
problem. I had emailed the spreadsheet to a friends computer, did some work,
saved it and sent it back to myself. When I opened it again the undo/redo
functions didn't work. When I first put the number in the cell before I hit
enter you can undo what you entered, but once you hit enter the icons turn
gray and don't work. I have researched every where I can think of, but still
cannot solve this problem. Your help will be greatly appreciated.

Saved from a previous post:

You could be using a workbook that has event macros. Most macros that do
anything destroy the undo/redo stack.

I'd start excel in safe mode:
close excel
Windows start button|Run
type:
excel /safe
and do a few things to see if undo/redo work.

If they work ok, then you'll have to find the workbook that contains the macro
that's doing the damage.

Chip Pearson has some notes on how to diagnose startup errors:
http://www.cpearson.com/excel/StartupErrors.htm

And Jan Karel Pieterse has more notes:
http://www.jkp-ads.com/Articles/StartupProblems.asp

=======
Another thing that could have gone wrong is that a registry entry was tweaked.

There's a setting in the windows registry that tells excel how many undo levels
to keep track of--maybe it was set to 0.

xl2k and above
XL: How to Modify the Number of Undo Levels
http://support.microsoft.com/?kbid=211922

xl97
XL97: How to Modify the Number of Undo Levels
http://support.microsoft.com/?kbid=162944

NEETZ123 wrote:

I have used Exel for many years and never experienced this problem before.
One day I opened a file and it was okay; I could undo and redo with no
problem. I had emailed the spreadsheet to a friends computer, did some work,
saved it and sent it back to myself. When I opened it again the undo/redo
functions didn't work. When I first put the number in the cell before I hit
enter you can undo what you entered, but once you hit enter the icons turn
gray and don't work. I have researched every where I can think of, but still
cannot solve this problem. Your help will be greatly appreciated.

--

Dave Peterson