The LCD screen turns whitte due to this reasons..........
1. Loose connection between the video cable and the LCD screen.
2. Defective LCD screen.
3. Defective motherboard ( assume the graphics card is integrated into the motherboard).
You have written that you listen the start up sound so your mother board is ok!
You can fix the problems by checking those cables......If you are not welled known better to go to the repair center.........
Best of Luck!!!!!!!!!!!!