Lights are out

My Frigidaire Fridge Model LGUB2642LF2 is having problems with the internal lighting. Both upper fridge and lower freezer lights all went out at the same time. Have removed the lights and tested with a continuity tester and they are good. Also, one of the upper lights appears to have melted the roof of the Fridge open from what appears to be excessive heat. What would be the most probable cause for them to all go out at the same time? Have snapped the switches for all the doors and no result.