I checked in the basements, but didn't see any leaks or obvious broken pipes. The basements are heated anyway, so it's unlikely anything down there is frozen. Checked the circuit breakers, and the board is green. I power-cycled the water-pump circuit, just to be sure.
I popped the manhole on the well and peeked down inside. It's really bright and glaring outside, and my eye has been bothering me again, so I couldn't see very clearly, but the pressure gauge on the supply line appeared to be at 0. I couldn't see any leaky pipes, either. I'm not about to go climbing down into a narrow, deep, dark hole by myself to make sure, though.
So either the well has frozen solid (fairly unlikely), it's dried up completely (also fairly unlikely) or the pump is on the fritz (possible).