For some odd reason best known to itself, the mysql process decided it was no longer going to respond to anything. Even (for those in the UNIX know) a
kill -9 signal. Usually kill -9 instructs the OS to simply remove the offending process, give it no more process time and clean up its memory.
It failed - which needs investigating because the only solution was a full reboot of the whole box, meaning Jabber, web, mail etc was down.
However, the situation lasted, I believe, for only about 20 minutes and should be back now and functioning, though the web server is getting the living daylights pasted out of it right now.
I shall monitor the situation and attempt to find out what went wrong, but at present it looks like MySQL committed suicide in a big way.
Steve.