Sorry about that. The hosting company restarted the box at their end without warning me and for some reason the server software didn't restart automatically. The restart took a grand total of nine minutes, but altogether the forums were down for almost an hour and a half
I'll restart it myself when it's quiet sometime and make sure I've fixed that, since I thought I'd told it to run at system boot. I've also been planning to put in a script to let me know if the server software isn't running when it should be, so I should get that done too (again, when it's quiet, as I'll want to test it by stopping the software).
Meanwhile, I've signed up to get an email next time before they do it - turns out they post scheduled maintenance on a particular page, but you have to opt-in to receive emails.