Hi, with the information you supplied i'm going to assume there is an ice build up at the rear of the freezer. Your problem is going to be with your defrost system. I will also assume you have the ability to repair this yourself. This is going to be one of three problems with your refridgerator, 1st thing to do is remove shelfs and panel in the back of the freezer...you should see ice build up on the evaporator...don't worry get a hair dryer and melt the ice. Next locate the wires for the heater...using a multi meter test the heater for condinuity...if the heater is bad replace it, If the heater is good replace the defrost thermostat..the stat normally is a barrel looking part clipped onto the evaporator pipe. The last possible problem could be the evaporator fan, have you heard the fan running in the past few hours?..if you are not sure pull the panel out to where you can see fan and watch it over an hour to see if it turns.

Take care