Most modern refrigerators defrost automatically. They have a defrost timer, a defrost heater, and a defrost terminator. When a frost-free refrigerator is running, it builds up frost on the tubing in the back of the freezer wall.
Normally, the defrost timer automatically cycles the refrigerator to defrost for about 20 minutes every 6 to 8 hours—it essentially heats up coils to melt the ice. If it isn't working right, ice remains on the tubing and, eventually, restricts airflow through the refrigerator.
On some refrigerators, you can manually advance the defrost timer, a small black box that may be located almost anywhere but is usually on the ceiling of the fresh food section. Advance it until it clicks into the defrost mode (you should be able to hear the heaters come on).
If this works, the defrost timer is defective and will need to be replaced, but this action will keep it running for a little while. If this technique doesn't work, the problem is probably the heater or the terminator. Call the manufacturer for advice.
