Can you post the model number please.
What you will find is the fault is inside the freezer. As all the cooling is done inside the freezer and a fan blows the cold air up into the fridge section.
Theres 2 things you can try here. The first is a total defrost of the appliance for 24 hours with the freezer door open.
Or empty the freezer and remove the back panel in there. From there you will see the evaporator. If its blocked up with ice then you will need to defrost it with a hairdryer.
Once its defrosted retry the applaince and see how it goes from there.
But if it does it again say 3 weeks later then theres a fault on the defrost side so you will need to test the defrost heater with a multimeter and see if its faulty.
Check it out and see what you find.
If some ice fall out from there then there may be a bit more blocking the vents up.
So just see how it goes after the defrost and let me know.
Can you hear the fan inside the freezer running?
Ok if the fan isnt running then this is proberly the fault. In this case the fan will need to be checked out with a multimeter and if faulty then it will need to be replaced.