What do we do when lights stop working?

I own Kenwood microwave oven MW786 with 30 litre capacity and microwave of 1000 watts power. It has five (5) microwave power levels and ten (10) auto menu programmes.Few days back its light stopped working after making a few packets of popcorns.But after after a few hours the lights came back. But the next day the light went off again while I was cleaning it with its plug still in the socket.