I am presuming that you have entered the correct security code for your wireless connection.
Either the "wireless network connection" is disabled in Networks Connections menu or the 'driver' which detects the wifi is corrupted or uninstalled. You can check whether the drivers of wifi connection is installed or not from 'Device Manager' and under that in 'Network Adapters', a wifi driver must be present.
You don't say the make and model of your laptop but you should go to the manufacturer's website and look under support for the wireless driver for Vista. Download it and try again.