Check your TCP/IP configuration. Ping your own IP address, then ping the access point's IP address, then finally ping the default gateway. If you cannot ping the access point, verify the association. If you are able to ping the above but unable to browse the internet using a web browser then the problem is most likely caused by something in the upper layers, like dns. More information can be found at Internetworking Troubleshooting Handbook.
If the configuration is valid, then check the routing capabilities on the network.
Intermittent connectivity - flapping
WLAN adapters (wireless card) / ACU (Aironet Client Utility)