Microwave does not heat/cook

Question

Yesterday my microwave -Whirlpool MH6150XMS (built in above stove) would not heat food. Everything else works fine, but the food is not hot when it comes out no matter how long it heats for. It is just 3 to 4 years old. It had a new motherboard about a year ago because lightening apparently struck it and the fan would keep running once the microwave finished a cycle. Suggestions? How hard is this to fix on a built in unit, or would it be best to call a repairman? Thanks.making a loud buzzing sound and won't heat