freezer evaporator fan not staying on

The freezer is cold on the bottom and the fridge is cold only on the bottom- but not as cold a s normal. The back cover was removed and the coils were covered in frost. Allowed to thaw and the evaporator fan ran for a short time. Does not turn back on. Replaced the defrost thermostat. Fan still not turning on. It does run, but only when it first gets plugged in and is cold. Help…

Check the voltage across the fan.
If it is 120 volts and the fan is not running, replace the fan.