Right-click context menu is missing on Windows 11
Assuming you've already restarted explorer.exe via the right-click context menu in Task Manager (or have rebooted Windows), this is happening sometimes on Windows 11 for reasons which I still trying to find out.
What might help in those cases and when the classic context menu is preferred, is to manually register the shell extension from a command prompt with admin rights:
cd <path to Mp3tag installation>
regsvr32 Mp3tagShell.dll