If you do an internet search, you'll find some good ideas for troubleshooting your problem. And you'll have some knowledgable guys from this board check in with good advice for you. Our side by side did the very same thing as yours a couple of months ago, and with the help of the folks on here, I fixed it myself. The faulty part in ours was the thermostat that kicks off the defrost heater. It was stuck in the "open" mode and wouldn't allow the defrost heater to come on. It was cheap - mebbe $15 or so. There's a defrost timer that can go bad, and the defrost heater itself could be the culprit. You said the fan(s) are running. To check the defrost thermostat, you have to check continuity while it's iced up. It should have continuity while it's iced - no continuity when it's thawed. You will have to isolate it from the circuit to test it (clip one wire).
You'll get some good advice on this board - lots of helpful and knowledgable folks here (and I'm not talking about myself!).