You have one of 2 problems. Either a large ice buildup has caused the evaporator fan motor to stop or the evaporator fan motor has failed.
You will need to take out everything inside the freezer. Look at the back wall of the freezer compartment. Is there an ice buildup on the wall? If so, you will need to remove the back wall of the freezer to gain access to the evaporator coils and clear out all the ice buildup especially around the fan motor.
If there is no ice buildup, it could be that the fan motor has failed and will need to be replaced. The part number is 4681JK1004A
Mtr. Assm.,ref Fan, AP4439225
Hope this helps.