Make sure you haven't accidentally switched off the wireless.  Most laptops have a switch located on them that can turn the wireless network card on and off.  Locate this switch and make sure it is turned on.  If your laptop doesn't have a switch, it may have a keyboard shortcut to do the same thing.  Usually the button will be located on the Function Keys (F1, F2, F3 etc.) and will most likely look like an aerial and may have some waved lines emitting from it, but you will probably have to hold down the Fn key to initiate the correct function of the key.