Update: I had looked on several discussion forums and came across one describing a similar problem, but with a different make tractor. Their solution was to park the tractor nose down on a steep grade for a couple of days and let gravity do the work. I had plans for the weekend that didn't include the tractor, so that's what I did. Sure enough, today I started it up, actuated the remote and 3-pt, and everything works fine.
Recalling what I did a few minutes before it stopped working, I was mowing on a steep tank dam; I guess it was steep enough to vent air into the hydraulic return.