broken f/freezer

Hi this is roy we have a liebherr fridge freezer CUN3031 service number 0902988/06, serial no is 42.699521.1
This fridge freezer is approx 3yrs old if we go back 6 weeks we had a problem where contents of the freezer had started to thaw. Best info I could get on the problem was a possibly we had accidently left the freezer door open which had caused an ice build up in the back of the freezer. We took the advise to swirs tch off the fridge freezer for 48hrs then restart it which we did. This seemed to sort the problem.
We have now had a second event of the freezer thawing and the fridge feeling warm and no matter what I do with the thermostat it doesnt change the inside temperetare.. I have been monotring the f/freeser for last 12 hours and I dont hear the compresser starting but you can hear the fan in the freezer cabinet running but not constantly. It cycles approx one second birsts on and off. When you open the freezer cabinet door the fan stops, wait a few seconds and the compresser starts. Whilst the freezer door is open and the compressor running within thirty seconds to a minute the condensation on the fridge back panel will turn to ice in the fridge please advise roy