Apple Music is stuck on "Loading Music Library"

I attempted to download from my iMac (10.15.6) to an old iPod Nano today, something that has always worked before. A partial download did occur, but now I can't open Music on my iMac. All I get is a window saying "Loading Music Library." No menu with commands, no way to even shut it down except Force Quit. Just stuck for the last couple of hours.

I've used iTunes since the beginning, and easily transferred to Music recently. Never any problems. What's going on here? How can I fix it?