I switched to the tuner. It was on FM. I hit the tuner button on the remote again, then tuned in AM 1020 (KDKA, the most powerful AM station around, just to see how they sounded since it was after dusk and hence all the stations which drop power to protect clear channels in the evening had already done so). After a minute I heard enough to know it was all good, and hit the tuner button again. The player displayed "FM"... and AM 1020 was still on. Switched back, then back to FM. Same.
Ok, so I switched to the player. It was paused and I couldn't unpause it. Gave up, restarted the player and it was fine, and didn't happen again.
Very weird.