I had this problem a few months ago, there is an heating element, controlled by a switch, the switch can play up from time to time, ie not come on, I just carried on defrosting it myself, as call someone out can be expensive, and sadly its cheaper to buy another ( sometimes) depending on the age of the fridge. However good news, mine just started working again for whatever reason? and as been fine for weeks now. But I would say get the hoover behind it and give the internal motor and switches a good go, because they do attract lots of cob webs and dust, inturn stop the switch working. Don't forget to power off first