I need help with this error message.

This particular file worked fine on Friday.   Now, on Monday, it won't open.  My other files seem to work just fine.

I get this error:

Excel cannot open the file "file name" because the file format or file extension is not valid.  Verify that the file has not been corrupted and that the file extension matches the format of the file.

the file is a .xltx file.  the xltx is all lower case.   I tried to rename the file, and point my template to the file, but that didn't work.  

Update: Copying a working template file and renaming it still produces a corrupt file.   However, I copied the working file, and then did not have the system rename it, I renamed it myself and then pointed to the template, and this worked.    I still cannot get the template that I need to work, to work.

Thanks.