Freezer doesn't stop running?

My in-laws recently had their basement back up and now they are having problems with their freezer. I went over there and began to look at it and noticed that the evaporator was completely iced over. We unplugged it and let it defrost. When we plugged it back in it ran constantly. Would this be a problem with the thermostat, controller or a relay. Also how would I check these areas. I also seen it could be an issue with the gasket as well. Does anyone have any suggestions on how to go about troubleshooting this issue. I'm also an electrician by trade but not familiar with the refrigeration process. If this helps it is a Gibson freezer model # gfu14f3hw. Thank you for your time.