A Square D breaker may occasionally fail and refuse to switch back into the "On" position. If the switch will not reset back to the "On" position with all the appliances on the circuit unplugged and all the lights switched off, then it's time for a replacement breaker switch. The circuit breaker replacement should be another Square D switch rated for the same number of amps. The hot wire connects to a screw terminal and the breaker plugs into the electrical panel.
Shut off all the breaker switches for the individual circuits and double breakers for the 240 volt circuits. Shut off the main power switch, which may be a larger switch located over the other breakers, or a regular size breaker which is labelled "Main," or "Main Power."
Loosen the screws on the outside of the breaker box with a slotted screwdriver. Pull the cover from the breaker box.
Pull the failed Square D breaker out of the panel to make room for the replacement breaker switch by gripping the back end of the switch at the inside of the panel and pulling it toward you. Pull the outside end of the switch from the mounting rail after the inside end is unplugged.
Loosen the screw terminal on the Square D breaker with a slotted screwdriver and pull the black wire out from between the terminal clamps. Slip the black wire in between the terminal clamps on the replacement breaker switch. Tighten the terminal screw firmly.
Snap the clamps at the outside end of the new Square D switch onto the mounting rail. Plug the back end of the breaker switch into the bus bar connector. Replace the cover on the breaker box and tighten the screws. Turn the main power switch back on, and then turn on all the other breaker switches. The circuit breaker replacement is successful if the new switch stays in the on position while lights and appliances operate on the circuit.
Only trained electrical workers are qualified to replace switches on an electrical service panel. Do not attempt electrical work without the proper training.