Remove your AirPort network passwords from the login keychain: Select the "login" keychain from the Keychain sidebar. Click the "Kind" column to order the list of keychain items by their type. Remove all entries of the kind: "AirPort network password".
Remove your AirPort network passwords from the System keychain: Select the "System" keychain from the Keychain sidebar. Click the "Kind" column to order the list of keychain items by their type. Remove all entries of the kind: "AirPort network password".
Note: The above steps will remove your Wi-Fi network passwords. If you do not know them, or if your network does not use passwords to restrict access, you should contact your network administrator.
3. Restart your computer.
4. Go to network settings / preferred networks and click the button to add your preferred network. Join your Wi-Fi network. You will need to enter your password for your network again if it requires one.
Done.
Click Sleep and wake it and it all will be fine!