You may have a spreadsheet that was attached to an email, or that was sent you some other way, e.g. from a website, I would guess.
Now, those can sometimes produce this sort of error.
- Try closing the Excel program, thus closing all other spreadsheets.
Then, Scan the spreadsheet's file, with your anti-virus utility. (If you don't have one, let me know, and I will advise and instruct you further.)
Now, after that, open just that one spreadsheet file. Does the error still come up? if 'yes', then you can try restarting your computer, then immediately after restarted and logged in again, try opening it once more. After that, I would consider the spreadsheet file probably corrupted or infected, and not try further. (And especially if you have no anti-virus program of comprehensive scope and good name, this would be inadvisable.)
Let me know how it goes, and whether you have an anti-virus utility, and I will advise you further.
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