If the LED is not on at all, it is likely that the battery is flat, or that the device is asleep. Measure the battery voltage with a multi-meter. If there is charge and you wish to see some activity reset the device by removing and replacing the batteries.
From power-on-reset, LED flashes for duration of start-up connect, fix, connect. Or until 10 min have passed.
After a reset of the device, the LED will flash. It will continue to flash until it has completed its start-up sequence of: connect to server (4Hz flash); get a GPS fix (1Hz flash); and reconnect to the server (4Hz flash). If the connections are not successful, this will timeout after 10 minutes.
After first sleep - off 
After the start-up sequence has completed (connect, fix, connect) or after 10 min, the LED will be switched off. No LED flashes will occur, until a power-on-reset
In Recovery Mode - flashes in trip
The LED will flash once every 5s during trips in recovery mode.