try hooking it up to an external display while off then turning it on. If the external display works then continue reading, if not then its your Motherboard (more specific its the graphics card) and must be replaced
IF the external display works, try reseating the RAM or using ram from another machine you know works.
Try remove any USB devices as well while trying both of these. There is a number of things that can go wrong like inverter, dead screen, bad video card, corrupted Bios... I could go on and on about how many things can cause this issue