turn off your modem and your router. leave off for at least 1 minute.
click on your airport signal at the top and go to open network preferences. click on advanced. click on tc/ip and make sure that dhcp is selected and then hit renew dhcp.
Upon doing that. click on assist me then click on diagnostics. it will give you a little screen that shows some red lights on the left.
Start up your modem and then your router.
the first three should turn green pretty quick. followed by the next three. If the next 3 stay red, there is an issue with isp or router. if anything stays yellow or you get an error that says self-assigned ip... then you are having a software problem and may need to delete your plist network files and start from scratch.