Make sure you're not running any kind of strange encryption on your router. I am sure the mac is quite clever when it comes to figuring out what kind of encryption is being used, but some older kinds might not work. I believe 'WPA Personal' is the ideal one to use.
And yeah it could be MAC filtered like that dude said, so check for that too. But if it's just a home network, I really doubt it. Talk to whoever set it up.
Unfortunately, wireless networking is a fucking mess and has a lot of different things that can go wrong with it... it can work one minute and not work the next even if nothing actually changed, or it can work with one computer and not another for no discernible reason.
I'd first restart the router and your computer, if you haven't already. Then go buy a gun and shoot yourself. I've been dealing with networks since I was a kid, but this year in my house I've had a network that just gets slow and stops working for no reason. I've replaced everything from the computer to the outside cable line and yet it still happens sometimes and there's really no explanation for it.