Frigidaire produces over-the-range, countertop and built-in microwaves. The latter are usually part of a wall oven unit. The units range from expensive, deluxe models to more budget-friendly options. Quick and convenient, microwaves are designed to be simple and safe to use, but like any appliance, they may break down occasionally.
Check the Fan
You might think that your microwave is still on because you can hear it running, but what you are hearing might be the exhaust system. Frigidaire microwaves have internal fans that cool down the microwave when it is too hot. If the light is off, the turntable is not rotating, and the display counter is done, chances are it's not actually on -- it's just running the fan to cool down. Units over a range or oven may automatically turn on the exhaust system when the area around the microwave becomes too hot. The exhaust system cannot be shut down until the temperature returns to a safe level.
Open the Door
If pushing the stop button does not turn off your microwave, try opening the door while it is still running. This should stop the operation of the microwave. If it does, the problem likely lies with your control panel, not the cooking unit itself. If the microwave continues to run even with the door open, quickly shut the door and unplug the unit. Then, call a professional to repair the unit.
Check the Turntable
Like the fan, you may think the microwave is running because the turntable is moving even after you push the stop button. It could be momentum, or it could be that the turntable motor is malfunctioning. To be certain, have the problem diagnosed and repaired by a professional.
Circuit Board Issues
A malfunctioning circuit board may cause the control panel to run, the light to turn on, the fan to function, and the turntable to rotate -- all signs that the microwave is working -- but the food may not be actually heating up. In this case, the microwave is indeed stuck in the "On" position, even if nothing is actually being cooked. As with the other issues, this problem should be solved by a professional.