Not sure what went wrong yet, but the server that runs the forum crashed/became inaccessible. A quick prod of the virtual remote reset button and things are back to life. All I know for now is that the disks aren’t full. A more detailed examination of the logs might tell me more.
And the answer is ‘Unknown reasons’. I see nothing in the log that would account for the server being inaccessible. I suspect it was actually still running but networking had shut down for some reason (nothing in the logs).
I’m just about to carry out a software upgrade which needs a server reboot, so things will go offline for a short while in a few minutes.