Try the basics first:
Change your router channel number. Most times this works & is all you have to do.
Disconnect & reconnect your modem. unplug it for about 10 seconds. Plug it back in. Do the same for Apple’s routers. Wait for everything to reboot.
Click the Assist Me button.
In the next window that pops up, click the Diagnostic button & do the necessary.
Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer. See these basic networking KB Articles: http://support.apple.com/kb/HT1401 AirPort troubleshooting guide
http://support.apple.com/kb/HT2712 Using network locations in Mac OS X
http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting
Also, run the Airport Utility app which is located inside the Utilities folder.
If using Apple's Airport, please re-post over in one of the AirPort Forums.