There are several things that can cause this issue. lets start with the easiest first . Static discharge. to fix this remove the battery and the power cable so the machine has no way of powering on then hold the power button down for 60 seconds then release it and put the battery back in and plug it in then reboot. If this gives no joy try connecting the laptop to an external monitor via the vga port on the side of the computer. If again you experience no Joy open the back panel that holds the memory and remove one dimm of mem try to boot again with just one dimm .repeat and reboot with the other dim in this will tell you if you have one bad stick of memory . If you still have no joy after all this then the problem could be in the GPU compaq had issue of heat causing the solder to break and loosing connection . If this is the case then we can try to reflow the solder on the board but this is a last resort . Try the other ideas first. Hope this helps