My hosting service did a server upgrade and it seems to have caused problems for a few sites including the board (but oddly not subdomains of cavaliertalk like the SM site).
Not really sure what happened but it seems to have swapped server spaces -- the board files were over in the wrong section of the server. At any rate I needed to do a minor board upgrade anyway so once I realised this was the problem I just uploaded the latest software version to the right section and the board reconnected to the database. Strange! I still need to sort the smilies.