Wifi working but no Internet??

Hi, my wifi is working but Safari tells me I am not connected to the Internet.

iPhone works fine on wifi (I am using it to type this!) a windows pc works fine and a windows vista laptop has no problems. Having looked at many old discussions on here I tried deleting airport and adding it again but it didn't work. I tried changing file sharing preferences I logged into he router to ensure the password was correct and it is (though network diagnostics says "this computer was unable to join the airport network you selected. You may have entered an incorrect password or network name, chosen the wrong type of wireless security or you may be out of range of the base station"

I haven't updated or changed anything. The computer hasn't moved from my desk--it worked two days ago by now it won't. I also tried moving system configuration folder to trash but that didn't work and my attempt to ping 74.125.127.99 as suggested by user BDaqua in an old discussion came up with

Ping: sendto: No route to host

Ping: sendto: No route to host

Ping: sendto: No route to host

Ping: sendto: No route to host

Etc.

I'd appreciated any help on the matter!

Thanks!