Sometimes - not always - a problem like this is caused by your router. In most instances, I suggest that folks unplug and/or reset (generally by unplugging the devices for 5 minutes or so) both the cable/DSL modem as well as the router. Then power up the modem again and then the router.
After you've reset both modem and router, delete any wifi preferences under System Preferences>Network. Then use the "Assist Me" button to create a new wifi connection.
If that doesn't work, call back...