Excel not working after updating to Big Sur

Your 2014 iMac cannot run a macOS newer than Big Sur, so the newest releases of Office aren't supported.

You might look into using Apple's Numbers, Pages and Keynote apps. They can open the Excel, Word and PowerPoint documents respectively.

You might also visit Microsoft's support site for guidance on why your current versions of the Office apps refuse to open your documents. Microsoft Support

And you might try starting your Mac in safe mode to see if that has any affect on the issue.

How to use safe mode on your Mac - Apple Support 

Safe mode will often correct weird software behavior by forcing the OS to do disk repair, clear caches and other housekeeping and temporarily disables third-party mods. When you're in safe boot, the machine won’t be at its best performance, especially with graphics, but that's expected.

Does the same thing happen while in Safe mode?

You can exit safe mode by restarting your Mac normally, then reevaluate the problem.

Exactly which Mac model and year are you using?

Find that info by clicking the Apple menu () at the top-left of your screen and then select About This Mac. (Note- for privacy and security reasons, please never post your serial number.)

What version of the macOS were you running prior to your upgrade to Big Sur?

Microsoft has a policy of supporting only the newest three versions of macOS. It's inconvenient for many users, but it is what it is.