start > control panel > network connections
if it happens again, first thing do this.
Go to your Device manager and look for any yellow ? or ! under the network adapter drop-down.(+ sign)
If you see a unknown device, or unknown network adapter, its most likely the wifi driver is buggered. Usually can just right click the yellow ? or ! and select update driver and let it look for the driver automatically. If it doesn't work, just remove the old drivers from add remove programs in the control panel and re install it from the laptops manufactures website.
If the Device manager is OK, and you see the wifi adapter fine, go into the network connections in the control panel and see if its there, disable and re enable.