I can't believe it. Almost the exact same thing happened.I also freaked but then I shut off the main pump to the waterfall and the water level is good. I'm with you in fixing it in the spring. Too cold now. I want to had a strem next spring so maybe we can talk then. II am actually having the exact same problem. I came home and my water level was less than half full 3 1/2 foot deep pond. I freaked!!! Then after I gained my composure I started looking for a leak in the stream and water fall and nothing. I refilled the pond and shut down the stream and water fall and crossed my fingers. I patched the liner toward the bottom when I first set up the pond 2 years ago and my biggest fear is that it came undone. lucky me after 24 hrs the pond is still holding water like a champ so its gotta be in the stream or waterfall. Being that the next day I can work on it is sun and we are expecting 40 deg and flurries I don't think that I want to be paying in the pond in that cold of weather. Im just going to add an aerator and deal with it in the spring time. So I guess the moral of my longwinded story is don't freak out. 90% of leaks are in the plumbing/features. If the pond holds water with everything off you in a VERRRRY good spot comparatively. The aerator will buy you time till the spring when you can get back out there and fix it.
Good luck