Excel 365 suddenly won't open specific worksheets?

[Lluewhyn]

I'm posting on behalf of my boss. He recently upgraded to 365, because his 2010 version didn't have some of the formulas that I use in my spreadsheets. Starting last night, clicking on existing Excel files will simply open up a blank Excel without any worksheets active. He then has to go to the File--->Open to actually open up the file he wants. Obviously, this is much more annoying than double-clicking on an existing file.

Any ideas on how to fix this and what may have caused it?

Thanks!