Hello tom-62,
Welcome to Apple Support Communities. It sounds like you're having trouble with the Notes app on your Mac after upgrading to macOS Big Sur. We are happy to share information that may help.
First, restart your Mac: Log out, sleep, restart, or shut down your Mac.
Next, start up in safe mode to see if the issue continues while you're in safe mode: How to use safe mode on your Mac.
If those steps do not resolve the issue, please let us know if you see any messages such as "Syncing" after you open the Notes app.
Take care.