It turns out I've had spammers registering accounts on this site for quite a while now. Fortunately, I require admin verification of any new accounts, so none of the spammers got anywhere. Unfortunately, I had failed to configure my email address, so I wasn't getting notified of any user registrations.
What this means is that I ended up with around 25000 registered users, at least 99.999% of which were spammers as far as I can tell. The end result is that I bulk deleted all inactive users, so if you attempted to register an account to post a comment or anything I'm afraid your account is gone. On the plus side, my email configuration is fixed and I've put a CAPTCHA in place so the spam registrations should slow down significantly, and I'll actually be able to activate any legitimate accounts going forward.
My apologies if your account got caught in the great cull of 2015, and I promise not to do that again. :-)