When the freezer stays cold but the fresh-food section warms, the sealed system is working. The problem is that cold air is no longer reaching the refrigerator compartment. Three causes dominate.
First, a defrost-system failure. In most modern refrigerators, the evaporator coil is inside the freezer; cold air is pushed from there into the refrigerator by a fan. When the defrost heater, defrost thermostat, or control-board defrost cycle fails, frost builds up on the evaporator until it's a solid wall of ice and airflow to the refrigerator stops entirely. Pulling the back panel off the freezer and finding a block of frost on the coil confirms the diagnosis.
Second, a failed evaporator-fan motor. If the fan doesn't spin, cold air stays in the freezer and the refrigerator warms up over several hours.
Third, a blocked or broken air damper between the two compartments. The damper opens and closes to regulate airflow; if it's stuck closed or the controlling thermistor has failed, no cold air moves.
All three are routine repairs with in-stock parts — typically $250 to $450 total including labor. Don't keep running the unit: the compressor can overheat trying to force the refrigerator down to temperature.