What do the Apple OS X network diagnostics report:
> System Preferences > Network > Assist Me... > Diagnostics ...
about your network connection?
Do the networking problems here also effect connections via a wired network?
Have you tried a different wireless network? Does connecting at a library, café or Apple store work?
I'm going to assume that you're not running OS X Server 10.6 on this MacBook — that would be unusual — and that you're probably running the client version of OS X 10.6, and are here in this forum because you saw the 10.6 in the forum title. If I'm wrong here, there can potentially be other triggers for the network outage. OS X Server management is rather different from whe OS X that Apple installs on a MacBook.