This will be an ice build up at the top stopping the air flow around the freezer. Wht happens alot on these is the drain hole blocks with ice and this causes he water not to drain away so it builds up as a block of ice and stops the fan from running.
What you need to do is either a full defrost on it for 24 hours and keep the door open. As this will allow the ice to melt fully.
Or you need to empty the top half and look at the top plastic tray at the front either side. There you will see a screw. Remove that screw and pull it down and hopefully you should be able to remove it if its not a big block of ice.
Then melt all the ice in there and then at the back you will see the fan behind the evaporator. You need to clear the ice in there as well with a hairdyer.
Once its all clear turn it back on and see how it goes from there. Fingers crossed it was just a blocked drain hole and that will get it working again from there.
This is a fault we see alot on these freezers so im quite sure the defrost will cure this.