The first step in fixing Christmas lights on a prelit tree is to find out where the problem is. Start by checking the wires and the bulbs for any damage. If there is any damage, replace the bulb or wire. If there is no damage, check to see if the fuse is blown. If the fuse is blown, replace it with a new one. If the fuse is not blown, check the circuit breaker and make sure it is in the "on" position.