OK, this would either be a fault in the defrost element or a blocked drain hole under the evaporator.
You'll need to find somewhere to store the frozen food.
Once the freezer is empty, unplug the appliance.
It will need to be fully defrosted.
Either 24 hours with the doors open, in a warm room, should be enough.
Alternatively, at the top of the freezer compartment, take the cover off of the evaporator and use a steamer to fully defrost the evaporator.
Refit the covers, plug the machine in and check it again.
This is the most common cause.
Less common is the door isn't sealing.
Check the door seal, see if it's damaged or failing to seal.
Check it out
Let me know what you find.