Both the Premium Heart Rate Monitor and the standard Heart Rate Monitor utilize a CR2032 battery. If a battery is placed in the device too soon after the old battery is removed, it can cause the accessory to experience a power overload causing the device to fail to operate.
To prevent a power overload for your accessory:
The heart rate monitor should now work. This process may need to be repeated if the monitor still does not display heart rate.
It may be necessary to rescan for your accessory after replacing the battery.
Premium HRM: https://buy.garmin.com/shop/shop.do?pID=15490
Standard HRM: https://buy.garmin.com/shop/shop.do?pID=10996