Clean the unit. A lot of air conditioning problems come from a dirty unit. Filters and condensers get dirty, as does ductwork. Either vacuum dirty parts or blow dirt off with canned air.
Replace the ductwork. Leaking ductwork can cause the unit to stop. Either replace the the ductwork or repair it by taping holes with duct or electrical tape.
Plug a voltmeter into a fuse to check the voltage. A blown fuse can cause the air conditioner to stop working.