Check Warranty? Some parts of the refrigerator may have extended warranty.

Likely one of two problems. Either the defrost cycle is not working or the cooling is not working.

Look for ice build up inside around the back of the unit.
If ice has built up on the back wall it is a sure tell that the defrost cycle is not working and the air flow is restricted by the ice built up around the cooling coils resulting in not cooling.

First try would be to empty the unit and allow with the doors open 24 hours with heat to melt the ice. Afterwards start normally and hope that the problem doesn't return. There is a tube that needs to thaw from the bottom of the coils that is known to freeze preventing a satisfactory defrost. This tube takes time to defrost if frozen closed.

If problem returns in a day or two then more troubleshooting is required. The heater, defrost sensor and depending on the model, timer may need to be replaced. This is far less expense then replacing the unit and I would be inclined to repair the unit.

No ice built up on back wall of unit does not mean that the defrost cycle is working so check for air flow coming out of the vents. If the air flow appears to be restricted it still could be frozen cooling coils and requires defrost.(see above)

If defrost is not the culprit than reread the warranty. It is likely the unit has a leak and the Freon is low or the compressor is failing. Check the fan located at the bottom from the rear or check for airflow from the front grill. Remove grill and check for massive lint and dust build up restricting the airflow before deciding to replace.

The cost of leak repair or compressor replacement can exceed the present value of the unit and could be better applied against the cost of unit replacement.

Hope this helps?