One of the circuit breakers has died: it tripped and won't reset. It's possible (but unlikely, I'm told) that it's a fault in the circuit rather than the breaker. In either event, fixing it will require that an electrician come out, which will apparently happen on Monday. They won't come any earlier than that unless it's a huge emergency.
This particular breaker took out the lights in the garage as well as the garage door opener. Like most, it has a manual release... which is fortunate, considering that I had the car parked in there.
I figure that this is a brand new house. We're certain to have some number of minor problems to work out over the next year or so. This is just the first.