Did you, by any chance, uninstall postfix or tinker with the database that would have been installed?
This happened to me recently when I was setting up a new server and messing around with the modules and ended up breaking Virtualmin.
Eventually I found a thread about running the virtualmin readiness checker and I realised that postfix wasn’t installed and needed to be.
Anyway, there was a precursor to it stopping so I suspect that has happened here.