How can I fix my BenQ Joybook laptop?

When I switch on the laptop, I get a blank screen. The power is on but it does not read the harddrive and Windows does not come on the screen. There is no light on the screen whatsoever. Then I cannot switch it off from the on/off button. This problem is only temperamental as it comes and goes without any particular reason. It has gone back to BenQ workshop and they could not find any fault with it. When it was returned to me, after about a week, the problem came back and then went away again!! Can anyone help?