I am having the same recurring problem, approx every 2 -3 weeks having paid almost £70.00 to have the freezer deforosted. Not helpful, I could have done that for nothing!
I have now discovered that leaving the fridge door open for about 10 minutes - until the internal light switches itself off - seems to kick start it so possibly there is a timer problem which I need to have investigated. Anyway, this appears to be a quick, if only temporary, fix so good luck.
Thumper, February 2011