Cannot Open O365 documents

***  This is a doc that I created a couple of years ago and accessed often.  Why would it all of sudden not open?  I do have other MS Office docs that still open, but I'm afraid to work on those and have them get corrupted (or whatever is happening).  

Thanks, Leslie

Btw - I don't have this problem if I create a Google Doc. 

You will get this message if you or some other program changed the filename extension.

If you take a .doc file and rename it .docx or .xlsx, that does not change the file structure. Same with an .xls or any other kind of file.

Google doc is not Word or Excel.

When posting, please be careful of the jargon. Otherwise you and others will be confused.

A doc is not a docx nor is it a workbook (xls or xlsx). They are all files.

Thanks Charles for clarification on naming.  I am not changing the name, only opening and closing it. 

Please try these options one by one and check after each option.

1. Try repairing your office installation (Quick repair is good enough). Try repairing your Office installation. First try Quick Repair (takes 5 - 10 mins) , if that doesn't help then try Online Repair (Takes more than 30 mins but less than 1 hour in general)

https://support.office.com/en-us/article/Repair...

2. Many third party applications also cause the problem like Tuneup Utilities, Abby Finereader, Kaspersky & other Anti Viruses etc. If you want to know if some program is conflicting -

Boot your computer in Clean boot mode and then start the application. If it works successfully, then some program is conflicting with this.
Refer to the section - How to determine what is causing the problem after you do a clean boot.
Don't forget to do - Reset the computer to start normally after clean boot troubleshooting.
https://support.microsoft.com/en-us/help/929135...

3. Sometimes, this is caused due to application not being able to access the default printer. Close all office applications and change your default printer through Start > Devices and Printers to another printer. It may be some physical one or One Note or XPS or PDF or anything else. If this is the problem, then you will need to update your printer driver for default printer.

4. Login to Windows with a new user profile and see if problem is resolved.

Do let me know if you have any more question or require further help. If above didn't work, I will supply more troubleshooting steps to solve this problem.

Thanks for the guidance Vijay.  The clean mode boot worked for one of the files that I couldn't open (a PPT deck), but I still can't open the Excel file.  So if it's an app that was causing the problem, how do I identify so I don't have to continue to use clean mode boot?  Thanks, Leslie