What router are you using?
What wireless adapter are you using?
Have you purchased the antenna as an optional extra or did it come with the card?
Does the same problem occur with other PCs or just the one?
What signal level is your machine reporting?
Have you considered using cabled instead of wireless? If not, why not?
The question in it's current form is a bit like saying "my car won't start, what's wrong with it?" It could really be all manner of things, and you'll need to be more specific by answering the above questions before anyone can give meaningful advice!