Ok this will be 2 things. Either the freezer evaporator is fully blocked up with ice stopping the airflow around the appliance or the compressor itself?
What you need to do is empty the freezer and from there you need to remove the back panel.
Behind there you will see the evaporator. If this is covered with ice you need to use a hairdyer and fully melt the ice in there.
Once cleared wrapped around the evaporator is the defrost heater. You will see the 2 black wires at the top.
You need a multi meter and test the resistance of the heater. If faulty then you need to replaced.
But if the heater is fine just retry the appliance and see how it goes. Fingers crossed its a one off but if it happens again and its all iced up then its possibley a faulty control board at the back here at fault.
But if after removing the panels theres no ice at all on there then its a compressor fault. For this it isnt a diy repair. You will need to get a fridge engineer in to replace this as you need to regass it once the compressor has been replaced. But this isnt a cheap fix as a new compressor fitted is around the £300 mark fitted.
So try the above and see what you find as it will be either too much ice on the evaporator or the compressor at fault.
If you need anymore info just get back too me.