The Ethernet cable may not be connected properly, your 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 your AirPort Extreme is
connected directly to the modem with an Ethernet cable 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. Click
Base Station Status to display information about the flashing light.
You can also select the “Monitor base station for problems” checkbox in AirPort
preferences. If the base station has problems, AirPort Utility opens and walks you
through solving the problems.