Excel was damaged after upgrading to MacOS Big Sur. Will not open. App is gone. I have lost important data. What can I do? Tried Time Machine backup but it’s not working. I am freaking out and really mad with Apple

Microsoft does not provide free upgrades between major versions of their Office products (unless you subscribe to Microsoft 365) and older versions of Office are not compatible with macOS Catalina or Big Sur.

If you have a version of Microsoft Office that is not compatible, you can either subscribe to Microsoft 365 or you can make a one-time purchase. See: https://products.office.com/en-us/buy/compare-microsoft-office-products-for-mac?ms.officeurl=mac-compare-redir

Or use the free and very good Libre Office https://www.libreoffice.org/

or the free Microsoft Office on the web: https://products.office.com/en-us/free-office-online-for-the-web

or Google Docs: https://www.google.com/docs/about/

or Apple’s Pages and Numbers

What MS Office version do you have and from what system did you upgrade from to Big Sur?

If you were running Mojave or earlier MS Office was probably 32 bit. With the introduction of Catalina 32 bit apps are no longer supported. You'll have to upgrade to 64 bit versions thru the developer or find an alternative.

For MS Office that would be Office 365 Personal, a subscription service or Office Home & Student 2019 , a one time purchase.

There are two, open-source, free alternative apps for MS Office: LibreOffice and SoftMaker FreeOffice 2018. Both can open, edit and save Office files in Office formats or their own format.

Your excel files should still be there and readable with the correct Office version or like others have stated with one of the free apps.