have you paired it with the garmin under settings? If so try pressing the reset button on the sensor and repairing.
I think you can do a differetn reset by fitting the battery upside down briefly and then putting it in correctly.
If it's still not working, try a new battery.
Failing that, is it nor recording anything? I had one that carried on recording cadence but not distance/speed - the reed switches are quite fragile and have been known to break on heavy impacts