POTENTIAL FIX
This just happened to me on a work computer - my file was saved on my work OneDrive and nowhere else. The computer installed an update overnight and this morning I had lost a months worth of changes though I know I use and save multiple times a day. Here's what I did:
In the altered excel file: File -> Options -> Save (in left panel)
I saw that my AutoRecover file location is set to C:\Users*Username*\AppData\Roaming\Microsoft\Excel - investigating this folder, I saw not one file related to the file I lost, so I looked to see if it was saved locally instead:
C:\Users*Username*\AppData*Local*\Microsoft\Excel\TemporaryBackupFile
and VOILA! Only one file in the folder and it was the last version of my file (last saved yesterday) labelled with today's date included before the file name. I just Saved As another version to my OneDrive AND my hard drive so I have more than one copy in case this happens again.
I hope this helps if it happens to anyone else again!!