The different lights on your Hue bridge relate to different things. So it depends on which one is off:
All lights are off:
Are you sure that the Hue bridge has power? Check the wall plug.
Lights 2 and 3 are off:
Check if your Hue bridge is connected to the Wi-Fi router. It needs to be hard wired into the router, using the network cable provided. Also, make sure the Wi-Fi router has mains power.
Light 3 is off or blinking:
Is your Wi-Fi router connected to the internet? You might be having trouble with your internet connection. Check if your other devices are online.
The big button in the middle is off:
Your Hue bridge can’t find your bulbs. First, you need to check if all the hue lights have power, via the wall switch or a free standing lamp plug. Then restart your app and follow onscreen instructions. If none of these option help, please contact our customer support for further assistance.