Check the power supply and fuse breaker if the freezer won't run at all.
Defrost the freezer if it isn't getting cold enough. Also make sure the temperature is set correctly, the door is closing and sealing, and the defrost timer is functioning properly.
Reset the thermostat if the freezer is too cold or too warm. If that doesn't fix the problem, replace the thermostat.
Check out the defrost heater, defrost timer and defrost limit switch if the unit won't defrost at all.
Test the timer by turning the dial until you hear it click. If you hear the compressor kick on, you have a faulty timer. If the freezer isn't defrosting properly and turning the dial turns off the fan or compressor, replace the timer.