An upgrade to the system broke the authentication system which DearDiary.Net uses and unfortunately took a lot longer to trace down than usual due to a complete lack of any error logging, coupled with a reboot attempt (which sometimes works but rarely!) totally failing because the server would not restart.
The server restart issue has been fixed, and won’t recur (at least not for that reason!) and I have recompiled the authentication module and it too is now working.
Apologies for the inconvenience