Well, I walked over to my tank and the pump was on the bottom of the tank and no longer attached to the mount or swivel arm. I must have bumped it while having my arm in the tank doing something. One of the tabs on the swivel arm was broken. It wasn't broken off, but it was weak and apparently not capable of holding the pump for more than a few minutes.
Thanks for the part number, I just ordered it from Tunze.com.