assuming yours has one compressor, and fans that blow the cold air around, a very common problem is the ducts, though which cold air is blown, getting choked with frost and ice and preventing the cold air blowing through.
Once they are completely blocked, the blockage gets more and more solid and, being inside ducts behind the back wall, difficult to thaw out.
A method that will probably work:
Take out all the food, and eat it, or put it in another FF
Unplug it
Pull it out from the wall if you can
Open both doors
Leave it open for at least 24 hours, or until water has stopped dripping inside, and into the evaporation dish at the back (sometimes walled in, but usually visible on top of the motor, with a tube to drain defrost water into the dish).
Mop up the water and clean out the dish, which may be full of dirt and mould. If the dish overflows, it may spill onto an electrical part and cause another fault, so sponge it out if you can.
When you are sure it has fully defrosted and drained, and is dry, you can plug it back in. If you plug it in before the ice blockage is all gone, you have wasted your time.
it may run OK for a year or two. If it blocks again soon, there is probably a fault in the defrost element or control circuit, which is an expensive repair, so consider the age and value of your appliance.
