MacBook Air - unable to reopen Excel document

I've been working on the same excel spreadsheet for months. I am unable to open it now even though I've just edited it last night. When I try to open it, a box pops up that says, "Alert. Excel cannot open the file 'Hours of RA Work.xlsx' 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." I do not know how the file has been corrupted or how to verify that it has not been. Please help! This document ensures that I get paid.