Could be all sorts of things, and unfortunately the easiest way to find out is to either replace parts one at a time or take it to a repair shop. You can probably write off RAM issues by taking all but one stick out, trying to boot then swapping sticks if it doesn't start. Presumably you're not getting any beep codes or flashing/different coloured lights when you start it up? It sounds like a motherboard issue to me, but that's just a guess.