GE JVM1870 Runs, but won't heat up. Less than 3 yrs old; the microwave will turn on & run

Question

Less than 3 yrs old; the microwave will turn on & run (normal sounds), but will not heat food (or boil water).i have 3 1/2 yr old GE spacemaker XL1800 microwave,it was fine until yesterday,now its not heating any food but still it runs if on it,please guide to repair it