I think they can lock up if you mount a lens that is not cocked with the body is cocked or something like that. Any chance you did something like that? The same think can happen to Hasselblad, or so I've read. I don't know how you solve that problem if it happens, however. It hasn't happened to me - yet.
And be sure your new battery is actually a good new battery. I recently had battery problems with my Mamiya and discovered my new batteries were old inventory and were dead when purchased.