White Screem after locked

I have a big problem of my iPod. After locked and when I want to use it, it appear pure white screen.

It still working 'cause I can unlock it and play music with it etc. as long as I remember where the icons is.

If I hold the home and lock for a while (i.e. reboot), it will show me the apple and after a minute, it works well as nothing happened. But as long as I lock it, it return to the white screen...

I believe that the screen has no problem. But I have restored it, just doesn't work. What should I do?