Hi again razvan!
OK, there was some system-wide problem at my ISP that was preventing me from accessing the Forums at all, but now that that has been fixed, it also appears that some database corruption might have resulted from my bungled attempt to change the $lang[WEL_MSG] text in the languages/english/adduser.php file.
Now, when I go to my Control Panel, I've got some mySQL error message - which includes "Can't open file: 'w3t_Messages.MYD'." - in the Received Private Messages cell of the page.
Furthermore, viewing the database via phpMyAdmin shows the w3t_messages table being flagged as "in use" and any any attempt to access it results in the following:
Database xxxxx - table w3t_Messages
Error
MySQL said: Can't open file: 'w3t_Messages.MYD'. (errno: 145)
Any suggestions as to how I can fix this will be gratefully received!