I'm not sure this should be in this category but when I turn on my self-build computer hooked up to my vga monitor, it say no signal when it is supposed to show the motherboard boot and window boot screens. I tried using an hdmi cable and is showed both of them. I am not sure if my computer is using on board graphics or the CPU's's graphics.
Make sure all the power cables between the power supply and motherboard are connected properly. Same thing happened to me recently. I forgot to connect the 4-pin ATX cable AND the 20-pin motherboard power cable. If everything looks correctly connected and it still doesn't work, disconnect everything and start again. Good luck.