Start by going to your circuit breaker panel and flipping the breaker that powers your AC. Likewise, turn the power switch that is connected to your AC unit outside to the off position.
Next, locate the little, red reset button. The majority of air conditioners have a reset button. These buttons are typically small and red. Consult your owner's manual if you're having trouble locating it. Most are published online for easy reference if you can't find it.
Now, press and hold the reset button for 3 to 5 seconds before letting go.
Lastly, restore the AC's power. Turn on all of the switches, including the one in your breaker panel.