I would try resetting your cable or DSL modem first, and then if needed then try resetting your wireless router. This fixes a high percentage of network outage problems in most residential setups. To reset a device, disconnect the power cord for 15 seconds and then plug it back in and allow it to boot up. To verify that this fix worked, go into the router config and check the IP address of your WAN/Internet connection. Valid External IP addresses should not begin with "0.0", "10.0", "10.1", "169.254", "192.168", or "255.255". If you have any other numbers sequence in the first two digits chances are that your router has a valid external IP address.

If that does not work, try reconnecting via the airport connection icon, and set it to the SSID (network name) of your router. You may be prompted for your password, WEP key, or WPA key if your network is secured. Enter it and you should be connected. If your are still having trouble, you might want to make sure your router is not using a default SSID (network name) like "Linksys", "Netgear", "Apple Network XXX", "Default". If this is the case, you could be interfering with a neighbors router, and I recommend setting a name for yourself and security key (WEP or WPA). You might also try changing the channel.