Ouch. Hopefully it'll heal relatively quickly. Move any perch/basking spot downward, basking light upward, and adjust wattage if nessesary. Might've been as little as a cooler day (ambient temp) that required him to stay under the basking light longer than normal.
If its not quite as warm at the basking spot, the lizard will simply stay a little longer to reach ideal temp. Better slightly on the cool side than slightly on the hot side.
Its a bit of a balancing game with artificial heat.
Best wishes