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/HT4628 Wi-Fi: How to troubleshoot Wi-Fi connectivity
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.
I tried the assist me/diagnostics. It said my internet connection appears to be working correctly yet the network status said failed on the last two radio buttons. Those being internet and server. The first 4 are all green.
I'm a little confused. My router is not wifi. I have an old Airport express connected to it. I used the airport utility and confirmed all the express settings. My wife's ipad and iphone connect and work just fine with the airport express. My imac connects to the express and shows a strong signal but there is no internet connection. The only thing I can do in the network prefs on the imac is click on airport, choose our wifi network and connect. Am I missing something? You turn wifi on for the ipad and iphone and it automatically finds the network and connects. Shouldn't the imac do the same?