Carbon monoxide is an odorless gas that is deadly if inhaled in large quantities. A carbon monoxide detector is designed to react to the presence of carbon monoxide gas and sound a warning -- providing an aural indicator of the gas' presence. A flashing green light on a carbon monoxide detector signals that there has been an electrical outage and that the detector is not receiving the normal amount of power it requires to function optimally. You can make the flashing green light return to a solid green within minutes, once you have located the fuse box that permits the flow of electrical power to the detector.
Go to the fuse box. Pull the fuse or trip the circuit breaker that supplies electrical power to the wall/area where the carbon monoxide detector is mounted -- the correct fuse/circuit breaker is named typically on a sheet mounted to the inside of the fuse box's cover or written beneath the fuse/circuit breaker. Let 10 minutes go by once you have pulled the fuse/tripped the circuit breaker.
Reinsert the fuse/trip the circuit breaker again to restore power to the electrical line. Let the green light on the carbon monoxide detector flash until it returns to a solid green color on its own -- if the light continues to flash after five minutes have passed, repeat the procedure at the fuse box.
Press the "Test/Reset" button on the side or front of the detector (depending on the make). Release the button after two seconds have passed. The detector will now "beep" once, flash a red LED once and then reset itself for use.