Excel spreadsheets missing on MacBook Pro M1, 2020 running Tahoe 26.3.1

Then the file was moved, renamed or deleted just as Excel says. Excel does substitute colons for slashes in a file name (likely because macOS allows a / in a filename but Windows does not). I renamed an existin Excel file to "Temp File 03/12/26" then opened it in Excel.

I then deleted the file (moved it to the trash) and tried opening it from Excel > File menu > Open Recent and get the same dialog you see:

So the bottom line is the simple explanation is usually right – Excel can't find the file, you can't find the file...so the file was moved or deleted.

Hopefully you have a backup and you can recover the file(s) from that. If not, I suggest that you learn the lesson from this and start backing up. Apple provides an excellent utility that’s already installed on your Mac, Time Machine. 

https://support.apple.com/en-us/104984

It’s recommended to back up to an external drive that is 2-3x the size of your internal storage. External storage is relatively inexpensive, a 1 TB SSD or a 2-4 TB HDD can be purchased for <$100. Consider that one backup may not be enough, a commonly recommended strategy is 3-2-1 where you have three copies of your data, two of which are backups and one of which is stored offsite.

As for your Gemini AI response, it sounds just as reliable as many responses that follow the GIGO principle – garbage in, garbage out. LLMs tend to mine user forums, and those are full of garbage.

Don't let AI generate macOS filenames for you. What you show is a horror show of file naming. Syntactically, it cannot be found by Excel or the Finder owing to its name construction.

If you want a date string in a filename, use the ISO 8601 format yyyy-mm-dd, or the non-US variant yyyy-dd-mm, instead of colons that the Finder will automatically translate into forward slashes. Forward slashes in filenames are only meaningful when you are in the Terminal and the UNIX command line. And then, some characters must be escaped. The Finder allows all other Unicode characters in a filename, though they should not be punctuated like sentences.

Using Spotlight, enter this:

name:.xlsx

and see what it displays for your Excel files. If you were saving files to Microsoft's OneDrive, I would also look there for missing files.

Also, looking at your screenshot the filename has colons in it. Colons ( : ) are not permitted in macOS filenames. How are you trying to open the file, from Excel > File menu > Open Recent?

Can you see the file when you navigate to that folder via your Desktop in Finder? If so, what happens when you double-click it?