Remove the covers from the evaporator coils in the freezer, so you can see them.
Manually rotate the defrost timer till the fan in the freezer shuts off
Do the coils defrost?
If yes, replace the timer
If no:
Defrost heater is open
Defrost thermostat is open
Contacts in the timer are shot
The above are the most common, there is also unusual things like a broken wire.
Freezer ices up 
