My upright frigidaire freezer stop running like defrost mode although the green light is on Is there a reset button for model LFFH20F3QM0.

SOURCE: Freezer won't run

There is no reset switch. There is a thermal overload protector mounted on the compressor. Does the compressor hum and then give a click sound. If so your compressor has failed. If the compressor is cold (Room Temperature) and it does not try to run your thermostat may be bad. If so remove the temperature control and jumper the two main wires that connect to the control. Unit should be unplugged to do this operation. Insulated the wires so they do not contact any metal surface. Plug the freezer back in. If it runs and cools replace the thermostat.

SOURCE: My freezer is stuck in defrost mode causing the alarm light to remain on

My freezer wouldn't run and I did find there was a reset button on the back of the freezer where the chord runs into the switch.

SOURCE: My freezer's defrost systems runs constantly

i dont think so i would look at the compressor kicking out on thermal overload or a faulty thermostat thermostats have two settings cut in cut out your cut in could be bad and not turning the compressor on till the temp reaches melting temps now if your hearing a click buzz then you need to look at the compressor and make sure the compressor fan is running but its not a defrost promblem

SOURCE: Frigidaire Upright freezer FFU20FC4AW2

Sounds like the timer could be getting stuck and not releasing the defrost cycle

SOURCE: frigidaire upright freezer MFU20F3GW7 (model)runs

Clean cooling coils....throughly. Check door seals.