All cooling takes place in the freezer (bottom, in your case). Freezer air must be circulated into the fresh food compartment to cool it. There is a fan in the freezer called the evaporator fan, which is supposed to do this job.
1. You have a blockage in the return vent to the freezer. Remove item that is on the floor of the fresh food section and find this return vent (hole or passageway) and prove to yourself that it is clear.
2. Freezer fan is not working. You should hear it when opening the freezer door. Some units shut opff when the door is open, so see if pushing the button that turns off the freezer light does that too. If not working find out why. Fan blade stuck on something? Motor seized? Replace failed motor.
3. Defrost system has failed, allowing ice build-up on the evaporator (behind freezer back wall). Test heater, defrost limiting thermostat, and defrost timer circuit. If a circuit board determines defrost cycle timing, replace it if the def. thermostat and heater are good.