iPhone 3GS not charging up

Hi. I have an iPhone 3GS (yes, I am still persisting with it). It was working fine all this while and then suddenly it won't charge up any more. I tried changing cords, chargers, plug points, hooked it up to different laptops/PCs, but still no go. I even rebooted the phone many times but even that didn't help. The power has now completely drained out and it's still not charging. How do I get it to charge up again?