The following is specific for the 11" 2011 MacBook Air:

If you are having difficulty with your Airport, especially after turning it off (for whatever reason you would), try the following:

1) Click on the Apple logo on the very top-left of the screen
2) Slowly go down the list and select [System Preferences]
3) Select [Network]
4) In the list on the left column, select the Wi-Fi connection
5) Remove the connection by clicking the [-] on the bottom of the left column
6) Click the [+] on the bottom of the left column
7) Add a Wi-Fi connection
8) Click [Apply]
NOTE: AT THIS POINT YOU ARE CONNECTED TO THE NETWORK YOU WERE ON BEFORE REMOVING THE CONNECTION, BUT THERE IS NO IP
NOTE: YOU WILL HAVE TO RECONNECT TO THE NETWORK



TIP: REPLACE [] WITH STANDARD HYPERTEXT PROTOCOL.
TIP: REPLACE WHATEVER IS BETWEEN THE [] WITH YOUR INFORMATION.



Hope that helps!