Addon Mappings Disappeared ....... GONE!

Hi Paul,
addition to Ian´s answers … for the next time, you can “backup/export” your mappings directly in the FDM on the “Mapping page”:

The “Export Mapping” creates a simple XML file, which you can import to a later time (new OS installation and/or as in your case, when something happens). Just a tip …

To your issue general:
Possible a virus-scanner or the windows-defender has deleted the file? The FDM itself, doesn´t make changes and/or delete the mapping files without any user-interaction.

But anyway, when you have re-mapped your addons, export the mapping and store the file on a safe location, that when something happens again you don´t have any additional effort.