First of all I would recommend creating a new network 'Location'. This can be done by following the instructions below;
Launch “System Preferences” from the Apple menu or Dock
Select the “Network” panel
Click on the “Location” pull down menu and choose “Edit Locations…”
Click on the + plus icon to add a new network location, give it a unique name, and click “Done”
This will create a new set of Networking settings.
We can also try trashing the Airport plist, this can be found in the following directory;
Delete com.apple.network.plist and com.apple.airport.preferences.plist
You may need to enable the Library since you are using OSX Lion, this can be done by typing the following command into the Terminal, found in the Utilities section,
chflags nohidden ~/Library/
Send this file to Desktop to keep it safe for now and then restart the Macbook. Now try to find your network.
Hope this helps,