There is a small LED light on your Arc (see #6 in the diagram below).
Blinking Red means your Arc is waiting to pair to your smartphone. Simply go into the KATANA Safety App and from the home screen click the menu button on the top left. This will open the menu screen on the left, from there select "My Devices” and pair your Arc. See "How do I manually pair my Arc to my App” FAQ for more details.
Blinking Blue means your Arc was connected but the Bluetooth connection is lost (possibly because you turned off Bluetooth) or that you have turned off your location services. Make sure Bluetooth is on and then, simply open the app and it should automatically re-pair after a couple seconds. If it does not, select the menu icon on the top left, select "My Devices” and pair your Arc. See "How do I manually pair my Arc to my App” FAQ for more details.
Note – The Arc is set-up to blink once every two minutes if it is not connected to your smartphone via Bluetooth or if your app isn’t running on your phone. This blinking is to ensure you know that there is no connection.