Excel file got "corrupted" / "tampered" on macbook

[joyapco]

I got an important password-protected excel file that Excel suddenly refuses to open, citing the file being "corrupted" or "tampered". 

I'm encountering this message first:

"Alert

This file may have been tampered with or corrupted and the contents should not be trusted. Do you wish to continue opening this file?" (Yes/No)

Followed by this after I selected yes:

"Alert

Excel cannot open the file [filename].xlsm 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've already tried changing the extension to .xlsx, .xls, html, .xlm but none of them could resolve the bug. Any other helpful advise is appreciated.

[#2]

@joyapco 
I'm inclined to believe the warning message. 
Is it safe to assume you are using version 16,65 of Excel? Go to the Menu Bar and choose Excel > About Excel to verify this.

Are there any special characters in the file name?

I recommend that you contact whomever provided the file to you and get a better copy.

[#3]

1) My Excel is 16.65

2) No special characters in the file name

3) I'M the creator of the Excel file. It's been working for 6 YEARS and it suddenly says it got corrupted. 

Since I posted this, I had to recreate months of entries from scratch and managed to recreate the file again. 

[#4]

You should be able to restore the previous working version depending upon where you saved the file. If it's in Mac OS then you can restore using Time Machine: If it's in SharePoint or OneDrive you can use Versions. If it was saved on a different file service, most file servers these days offer some sort of recovery system.