If your BSXinsight is charged and the LED light is off, it's in a "sleep" state to save power. It can last 4-5 days like this before the battery runs out. There are 2 ways to wake it from the sleep state:
METHOD 1 - Put it in the dock/cradle (that's powered) for a second, the LED will turn blue, then pull it out and it should turn green.
METHOD 2 - Tap it a few times with a finger, until the LED turns green.
Once it is green, it will accept a Bluetooth 4+ (BLE) connection from your BLE phone that has the BSXinsight app on it.
*NOTE* if the LED is blue when not in the dock, it's connected to BLE already. If the app is open, it can auto connect to a BSXinsight it has previously connected to.