First of all, you would need to have remained connected to the network for a considerable period of time for a complete image of your hard drive to be built and transmitted. If your system drive is just 40GB it would take somethng like two hours to transmit a full image over a wireless connection running the most common 802.11g protocol so I would be very surprised if that had happened.
Whatever data may or may not have been accessed you can't reverse now but you can try to ensure that you are protected in the future. Go into network settings and check what networks you allow connections to and what sort of connection is permitted. Make sure that access is password protected, you share only files and folders which you need to and...