Smoke alarms are designed to sound off when smoke is detected. A smoke alarm that doesn't reset itself after a power failure and instead sounds an alarm must be turned off. There are a number of procedures for turning off the smoke alarm so it is returned to its normal operational state -- none of which permanently damages the device. No special tools are needed.
Locate the "reset" button -- this can be on the smoke alarm's control panel or on the smoke alarm itself. Press and hold in the "reset" button until the alarm ceases. Release the button.
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Remove the smoke alarm from the base by twisting it counterclockwise, if the alarm continues to sound unnecessarily. Remove the battery that's no longer providing enough backup power to the smoke alarm from the battery compartment. Place a fresh battery into the battery compartment. Twist the smoke alarm back onto the base.
Remove the smoke alarm from the base again, if the alarm has still not reset. Wipe out the inside of the smoke alarm and the vents on its side with a water-moistened paper towel. Dry with a paper towel. Blow off the sensor inside the smoke alarm with a burst of compressed air to remove any loose dirt or other contaminants fouling the sensor plate. Rub the sensor plate with the tip of a cotton swab moistened with distilled alcohol. Twist the smoke detector back onto the base to stop the alarm from unnecessarily sounding.