I solved this problem few times by completely opening my HTC Desire Z mobile phone and then reassembling it. Third time, I noticed that the problem is due to looseness of two, out of three, tiny screws at the "camera" button of the mobile set. Therefore, mobile set was not properly pressed and sealed at the "Camera" button and due to some protection by HTC, screen went black. Now I have used new relatively long tiny screws and the problem is solved permanently.