AirPort - Unable to connect after sleep/restart

If you find that your Mac is having trouble reconnecting to an AirPort base station by wireless after either it has been sleeping or after a restart, please try the following, in order, until (hopefully) resolved:

1. Create a New User Account
   • System Preferences > Accounts
   • Click "+" to add a new account.
   • Log off, and then, log back on with the new user account.
   • Try connecting to a Wi-Fi network with your Mac's AirPort.
   • If this resolves the problem, there may be something amiss with your current profile and will require further troubleshooting.
   • If this does not resolve the problem, go ahead and log back into your normal account and go on to step 2.
2. Delete AirPort Keychain Entries
   • Launch the "Keychain Access" application located in Applications/Utilities.
   • In the windows on the left side: Select login for Keychains and "Passwords" for Category.
   • Click on the "Kind" filter at the top, and look for any "AirPort network password" entries...and delete them.
   • Close Keychain Access.
   • Continue to step 3.
3. Delete Preferred Network(s)
   • System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
   • Delete all entries under "Preferred Networks."
   • Click on OK.
   • Continue to step 4.
4. Add Preferred Network(s)
   • System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
   • Add the preferred network(s) using the "+" button.
   • Click on OK.
   • Restart or log out, and then, back in.
     • If it works, then you are done.
     • If not, go on to step 5.
5. Move System Configuration Files
   • Quit any applications that are currently using Wi-Fi.
   • Turn off Wi-Fi from the Wi-Fi icon on the OS X menu bar.
   • Open Finder > Go > Go To Folder > Enter "/Library/Preferences/SystemConfiguration/" into the Go to the folder window > Click on Go
   • Locate and select the following files:
     Note: Files are OS X version specific, not all the following files are used in each version.
     • com.apple.airport.preferences.plist
     • com.apple.network.eapolclient.configuration.plist
     • com.apple.wifi.message-tracer.plist
     • NetworkInterfaces.plist
     • preferences.plist
   • Move any matching files to the OS X Desktop. You can place them all in a folder as a backup. The "missing" files will be rebuilt after restarting the Mac.
   • Reboot the Mac.
   • Re-enable Wi-Fi from the Wi-Fi icon on the OS X menu bar.