If you find via the Fusion Client Portal under Bonding Admin that one of more of your lines that you are Bonding is showing a 'red' icon, indicating it is down.
Then the best starting point is to:
1/ Verify that there are lights on the modem indicating there is power supplied to it
2/ If your Modem has power there should be light activity on the led indicators (rapid blinking every 30 seconds as a minimum). If there are lights on but no blinking then verify that the cables between the Modem and the Bonder are correctly inserted.
The Bonder should be showing either a green or orange light above the corresponding cable. If there is not, remove the cable and reinsert it. If still no lights appear try another cable between the modem and the Bonder.
3/ If the Modem is showing lights and there a rapid burst of blinking every 30 seconds and the Fusion portal still indicates the service is down. It may be a fault the line or possibly a ISP fault.