Your fridge does not make cold air on its own. A fan in the freezer moves cold air into the fridge through a vent. If that airflow is blocked, the freezer can stay cold while the fridge gets warm.
First, make sure food or bags are not blocking the vents. Then check the door seal. Clean it and look for gaps. A bad seal lets warm air in and can cause frost to build up.
Open the freezer, press the door switch, and listen for the fan. If it is quiet or running weakly, the fan motor may be bad.
See thick frost on the back wall? Ice may be blocking the airflow. Unplug the fridge, empty it, and let it fully defrost.
If the ice keeps coming back, the drain, damper, or defrost system may have a problem. Book refrigerator repair service to have it checked.