You don't perchance have one of those laptops with the heat problem? The graphics card heats up the motherboard; the solder in the WiFi bit melts, and the wifi stops working.
A known bug, solved only in the last six months or so. A friend of mine had one: while we were persuading the manufacturer to fix it (they did), he borrowed my WiFi dongle which worked fine. Try yours with a dongle if you have one.