GoDaddy (our current web host) did mention a month ago that we would "soon" be migrated to a new web server.
That was the last I heard from them, but plainly it happened last night, and took longer than they expected.
The whole website was down this morning, and even when it came back, the SQL database used by the forum software took about another hour to come back online.
So, sorry about the lack of warning, I had expected GoDaddy would get a bit more specific before actually doing it!