It’s possible the thermostat has failed but you’d normally expect everything to defrost as it controls the single compressor which cools both compartments.
Is the motor (compressor) running constantly or does it cut in and out ?
If it’s running constantly then a system failure is the likely cause I’m afraid, as long as it’s not the fan motor in the freezer you can hear.
An engineer calling is the only sure way of telling 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} of course. Check the repairs@ link below 🙂