I am sure you have tried this, but if not give it a try. The following is a copy of page 26 in the manual.
Your AirPort Extreme Status Light Flashes Amber
The Ethernet cable may not be connected properly, AirPort Extreme may be out of range of an AirPort network, or there may be a problem with your Internet service provider. If you are connected to the Internet with a DSL or cable modem, the modem may have lost its connection to the network or the Internet. Even if the modem seems to be working properly, try disconnecting the modem from its power supply, waiting a few seconds, and then reconnecting it. Make sure AirPort Extreme is connected directly
to the modem via Ethernet before reconnecting power to the modem.
For more information about the reason the light flashes, open AirPort Utility, select your base station, and then choose Manual Setup from the Base Station menu. The information about the flashing light is displayed on the Summary pane.