You may be out of range. Make sure your computer is within range of the wireless device you’re connecting to.
Check for sources of radio frequency interference, such as a microwave oven.
Restart your base station by unplugging and plugging in the power cord again. Then try reconnecting.
Make sure the Wireless Mode pop-up menu is not set to Off in the Wireless pane of AirPort settings in AirPort Utility.
The AirPort service might not be selected.
On a Mac: On a Mac, Open System Preferences, and then click Network. Make sure AirPort is available in the network connection services list. If it isn’t, click Add (+) and choose AirPort from the Interface pop-up menu. Give the AirPort service a name, click Create, and then click Apply.
On a Windows computer: Open the Network control panel and make sure the wireless network connection is set up correctly.