Well, the old "have you tried turning it off and turning it on again" might actually work here for your router. Once you've done that, wait a minute or so while the router starts up and then see if the wireless LED is lit.
If all appears okay with the router you need to investigate the wireless adapter in your computer -- see if it can detect any wireless networks -- assuming you live in an area where you have previously noticed neighbouring wifi.
Yes, reboot the router, if you still get no signal, go into your control panel, and under network connections, find the wireless adapter you are having issues with. Right click on it and disable it, to turn the wireless card off, then do the same thing again, and enable it again. Sometimes that will do wonders.
still no luck, my computer says that the local area connection cable is unplugged but that cant be right because my other compuuter is just fine, i just found out thet my network discovery is turned off, i have windows vista on my computer can someone help me out on this one?
Sorry for my earlier misunderstanding -- I thought this was wireless. Have you checked that the problem computer's network card is installed properly. Go Control Panel, System, Hardware, Device Manager and see what's listed or if anything is yellow flagged as faulty.