Excell file will not open

[walkerstoo]

Excel Office 365 (older files) will no  longer open

All have the .xlsx extension

I get error message" that the file format or extension is not valid or file is corrupt"

What can I do to resolve this?

[#2]

@walkerstoo 

It usually works if you close Excel and start again.

Note: Check the task manager (process tab) to verify that Excel (and Microsoft.Mashup processes) are really closed.