Undo won't work MS Word 365 on Windows 10

If you have Office Professional Plus 2013, it will be locally installed and you will need the Product Key to re-install it.  Make sure that you have that before removing your present version.

Before uninstalling and re-installing, I would try repairing the Office installation as detailed in the Knowledge Base article at:

https://support.office.com/en-us/article/Repair-an-Office-application-7821d4b6-7c1d-4205-aa0e-a6b40c5bb88b?CorrelationId=1b0030f4-8cc5-4485-b495-810e2a7d1284&ui=en-US&rs=en-US&ad=US

Also try starting Word in Safe Mode by holding down the CTRL key while starting it until asked to confirm that you want to start it in Safe Mode and see if the problem then exists.  If it does not, it indicates that there is probably an Add-in that it causing the issue.  I would also try renaming the Normal.dotm template to Normal.old and then restart Word and try again.

Possibly a dumb question, but is there anything to be undone?  When you first open a document, the Undo button is always disabled.  It is not until you have changed the document in some way that there is anything to be undone and the button should then become enabled.

It that was a dumb question, then, you might remove all traces of the Office installation by downloading the fixit from the following article:

http://support.microsoft.com/kb/2739501

saving it to your computer and then running it.

Then reinstall Office by logging into www.office.com/myaccount with your registered email address and click Install Office.

BEFORE REMOVING THE OFFICE INSTALLATION,  MAKE SURE THAT YOU ARE ABLE TO LOG INTO YOUR ACCOUNT AND THAT OFFICE IS ASSOCIATED WITH IT!

If you actually have Office 2013 (locally installed) rather than the subscription version provided through Office 365, then you can't get Office through your Microsoft Account. Instead, see https://products.office.com/en-us/download-back-up-restore-microsoft-office-products

The problem isn't "Why I can't start my office 2013 application." The problem is the UNDO button won't work, and the program keeps crashing and losing changes. Since Windows 10, Word (Office 365) has become jerky and unresponsive thirty or more times a day. The screen locks up. Hours of work are erased because it isn't recovering the files. And this is with my setting of autosave every two minutes. Even though, when I press the file tab, Word shows my file saving every two minutes, when the program "white screens", for lack of a better term, I know that I've lost everything I've done since my last hard save.

I have the invite to upgrade to Office 2016, but I'm scared the glitches there may be worse. If I can't get this fixed, and fixed soon, I will have clients whose edits are incomplete, therefore losing income.

Hello Lisa,

Welcome to Microsoft Community and thank you for posting your query.

You may start Office Word application in safe mode using the following link and check if it works as expected.

If Office Word works as expected in application safe mode, then disable the add-in(s) by following the steps from the link below and verify the status.

http://office.microsoft.com/en-us/support/why-cant-i-start-my-office-2013-application-HA104011864.aspX

Hope the information helps. Do reply if you need further assistance, we will be happy to help you.

Thank you.

LISA!!!!!!!!!!! Sorry, I'm a little excited. I didn't mean to shout. ;-) This morning, I posted my problem on the all-knowing FaceBook and got a response within minutes. I was advised to uninstall Office and then re-install. As of right now, I have my UNDO back. My CPU and Memory are still spastic, but for right now, I can live with the dial up flashbacks. Try it, 'cause it certainly can make things worse.

Ignore my excitement. It didn't take long for it to gray out again. Back to the drawing board.