Microsoft Office Lost when upgraded to Big Sur

Some older versions of Office are not compatible with Big Sur so you may need to purchase a newer version. Office 2019 (?) is the latest I think, but I don't know if Office 2016 is supported with Big Sur. Anything older than Office 2016 is unlikely to be supported.

Whenever you upgrade to a new major version of any OS you should always check that your third party apps will be compatible. Normally minor OS updates for security patches won't affect app compatibility.

Also always make sure to have a good backup before an upgrade to a new major OS version just in case something goes wrong or if you want to revert to the older OS because you don't like it or an app is not compatible.