Have the boiler inspected by a plumber; if the unit needs to be reset, it's likely a fault of some kind has occurred. Having the appliance examined rules out the chances of the problem recurring.
Locate the reset button on the side of the boiler. The button is usually grouped with the other control buttons and clearly labeled. The reset button is sometimes hidden behind a flip-down cover.
Press the reset button firmly. If your boiler is computerized, you might see a visual or auditory cue indicating the boiler has been successfully restarted. Basic or older systems simply reset without a light or sound.