I feel embarrssed for asking this but have you checked that the wireless adapter is not switched off -- some PC's have a fairly inacessible switch which can sometimes get turned off by accident.
Some machines will also have a small led when the wireless adapter is working.
If you can also get to a LAN or a Router try and plug a piece of network cable in to see if the built in network adapter (non wireless) is working correctly.
Also check in device manager if the wireless network driver
has a question mark against it assuming you can see the device at all in the network connections.
Macbooks are usually pretty foolproof so I'd imagine the error is bound to be something really silly.
Added -- some machines also have a SOFTWARE switch for wireless instead of the hardware one. If your machine has this then run the application to ensure the wireless function IS enabled.
In all the years I've been using computers I don't think I've seen a case of where a wireless network card has got physically broken so I'm sure the hardware is OK.