IPhone 3GS display is not working properly

I am using an iPhone 3 GS. It is a nice phone. But i am suffering from a great problem.
The problem started to show a white screen. And there is no way to reset it. I
tried to shutdown and then turn it on. Then i entered the password and then reset it, i can use it for a few moments.
And then the white screen come back again.
Please help me. how can i solve it?
