The temperature in the fridge compartment is set using a plastic slider with an embedded temperature probe. The slider is attached to one of the cooling fins in the fridge compartment. Is it possible that this slider came off of the cooling fin?
Make sure this slider is on the fin and that the temperature probe is attached to the slider. Then try sliding it up (in the cooler direction).
As was mentioned, opening the door can really upset the cooling. It would be better to get an in-unit thermometer with a remote display, like this:
This is what I use and it works really well. Plus, I can carry the display in the truck while towing to keep tabs on the fridge.