How many miles have you put on the engine since installing the rebuilt head and gasket set? Have you checked the oil for traces of coolant? Are there any other symptoms?

Odd that the car would run perfectly well one day and have a burnt exhaust valve the next but it is possible. Usually valves burn because they do not seat properly (the valve should seat flat to the head), which would indicate a problem with your new head. If it is indeed a burnt valve, you will have low compression in that cylinder and the engine will misfire or run unevenly.