Does the evaporator fan in an upright freezer run during the defrost cycle?
Check to see if there is 115 volts on the two leads (hot and neutral) supplying the motor. If there is voltage and the motor won’t run, the motor is bad. If there is no voltage, you have to find out where you lost it. The defrost timer does cut the motor out when the system goes into defrost, so that could be the trouble. Some units have a combination door switch which will turn the light on and cut the motor out when the door is opened. This switch could be bad. Also look for burned or loose wires. Good luck.