I have an Apple IBook G4 it was working fine with our internet, now it will connect with our Wifi but will not connect with the internet.
Check your wireless router to see if it is actually connected to the internet (look at the status lights). If the "internet" status light shows not connected (light flashing or not lit), contact your ISP to see if there is a problem with the internet service in your area.
Per chance... have you tried using an ethernet.cat-5 connection between the iBook and the router? Other possibles... power cycle the router (switch off, leave about a minute or so, power up again).