Simpad Sl4 Black Screen (but It´s Swich On¡¡¡)

Hi,
I am new in this war and i wonder if someone can help me.

I have a sl4 updated to win ce net 4.1 and all seemed to be ok until 3 days ago.. The problem is as folows:

The sl4 screen goes to off, but you can heard the tap noise on the screeen if you tap, and you can see the screen off ,like a shadow, so is on but without light.

I sent it to siemes and they replace the unit, all was ok. Now i try to use it after 3 years and after 2 days working ok the problem appears again.

I have to swch off and on again adn sometimes work. Sometimes wroks if just resets, but after a wile goes to off again.

This is a great challenge for the brains shown here. I have tried all you can imagine, but not success.

I have not toch or recplace any part of it so what can it be....

Thanks in advanced for your help.