The light had gotten hot enough to heat the caulk seal around the bulb and the vibration from the bumpy road shook it loose. Bummer.
Sometimes we have faith in something and I, like Kip below (last 16 seconds), learned that sometimes it lets us down.
Fortunately, by this time there was enough daylight so I didn't need the headlight for the last couple of miles. But still. Dang it!