DIAGNOSIS: Loss of vent suction. The damper (the flap that opens to the outside) is either stuck closed or the vent direction isn't configured correctly for your ducting.
FIX: First, confirm your venting setup. This model can be set to vent outside (requires ductwork) or recirculate (uses charcoal filters). If set to 'vent outside' but the damper is stuck, you'll get noise but no airflow. Check the external wall cap damper for obstructions (bird nest, ice). Then, inside, remove the grease filter and feel for airflow while the fan is on high.
IF THAT DIDN'T WORK: If set to recirculate, the charcoal filter is likely completely saturated and needs replacement. These filters are not cleanable and should be replaced every 6-12 months.
HEADS UP: Cooking on high heat without a functioning vent will coat the microwave's interior and electronics in grease much faster, leading to other problems like overheating and odor.