I don't know. I wasn't on when it happened so I'm just going to assume you're making this up. Everyone knows threads don't just "disappear".
Most likely it was deleted. I looked in the threads started by you and it wasn't listed. You either broke a rule or the mod thought it was pointless. I had a thread deleted a while back (I'm assuming) because it just vanished and I didn't even see the notification thing on the forum where it had been but when I went to threads started by me, it was gone. Apparently starting a thread asking to meet new friends in your city can be interpreted as asking for a hook up. *sigh* I think most everybody at one point will get a thread deleted.