Something caused the server to take a massive shit last night, which in turn caused it to stop functioning.
I restored from backup, but that was 10AM, so we lost most of what we all did yesterday... (sorry, I lost a fuckload of work too)
I am looking into more frequent backups... trying for every 4 hours at least.
For now, don't summon the Wither again, I will get on and do a backup right before the next attempt, just in case there is some kind of issue with that and some Tekkit mod or something.