Ok so the PC is still on when you lift the lid, but not waking up. There are two main possibilities. You may have a broken switch which most computers will use, to know when the lid is open or shut. You may also have a corrupt VGA driver. It is common for the VGA driver to become corrupt on some units and the weird thing about it is they work only until you wake the system back up from sleep mode. But usually that involves a flickering screen or a jumbled image, I have not seen a screen just stay black. My first guess would be the lid switch. But it could be other things as well.