My mother's GE microwave oven

Question

my mother's GE microwave oven has a large burn mark on the ceiling of the unit. She noticed some plastic parts on the floor of the unit. She bought the combination microwave-stove about four years ago and noticed this mark about ten days ago and has stopped using the microwave..... Her problem is that the thing is attached to the oven and she is afraid that she'll have to replace the entire unit...any suggestions?