Try to check your LCD connection that no pins are bent. Reseat the connector. That connector is very fragile and always be gentle when seating the connector as you can bend pins. Take a look at why I once lost backlight on a 4g Touch: https://www.dropbox.com/gallery/943721/1...
If it looks ok I would try to do a software reset. A lot of times you loose backlight or white screen if you reassemble and accidently have switched it on.