Insert the battery upside down for at least three seconds. That will reset the device.
Also check firmware. This is done through the Wahoo Utility app and so anyone who owns a Wahoo sensor should have the Wahoo Utility app on their phone. Just connect your sensors to your phone once in a while, run the app and it will prompt if new firmware is available.
https://itunes.apple.com/us/app/wahoo-utility/id359225351
-Tim-