Unfortunately it could be a variety of things. If you are CERTAIN you didn't spend it on something then check ALL of your characters. Do you have any PvP only characters that you often delete and rebuild? Would they have had the money when you deleted them? Did you, like October asked, join any guild websites? If so did you use the same passwords and usernames? Last question - have you experienced anything like this before or any other glitch-like effects from the game?
@Chris. I THINK you are joking but I'll make an debate anyways

I completely disagree because often we end up in guilds in which don't want to be in. I am always a stickler for joining a new guild. I am never sure if what I'm getting into will be what I want. This is why I say, "I am interested in your guild but I am currently very open to all varities, but looking for something in particular. If for whatever reason I decide to leave your guild I do apologize. In case this happens I would like to offer to pay the 100g invite." This makes it so I can feel guilt free, if they accept or don't, if I leave. I have had to do this a couple of times due to lack of interest in the guild after a week and another time when I decided to join up with old friends (my current guild.)