Any particular laptop maker and model number? Any particular operating system?
Even if the antenna were totally disconnected from your unspecified model laptop, the internal MiniPCI card would still pickup some signal at close range. Is that what's happening or is your wireless laptop totally comatose? If so, it's not the antenna. Look for a wireless enable/disable setting, button, keyboard combination, or well hidden switch on the side of the laptop.
Replacement laptop wireless antennas, with the correct connectors and cable lengths are all over eBay and various manufacturers web piles. However, you'll need some expertise in repair methods in order to disassemble the display, hinge area, and laptop base to perform the replacement. As I mentioned before, I don't think the antenna is the problem unless you have a weak signal.
If all else fails, you might open your laptop and check if the two antenna cables are properly plugged into the MiniPCI wireless card.
We can all recommend excellent USB adapters.. But, if you are using your built-in ,i.e., Centrino wireless then you might try it at a Starbucks or McDonalds before deciding that your antenna is bad. Next I would uninstall and reinstall the drivers..I would not guess that the antenna is the problem. Do you have A/B/G or B/G? Can you remove the access and see your MiniPCI card??? Etc...
If XP then go to "connect to," then "show all connections," then right click Windows, and ascertain that Windows at right click shows enabled. Then click "show available networks." Alternatively go to Control, System, Hardware, Device Manager, Network adapters and be sure that there is no yellow mark next to your wireless adapter.. I suspect that it will be working..Good luck..