Disconnect all the cords and then only connect the power for the screen and turn the screen on. It should come up with the brand name and then just disappear and say ' no signal ' or something and then if it says that we'll know it's not the monitor.
If you have a spare monitor try it with that and see if it works. If it still doesn't work you'll have to poke around inside the case. I had the same sort of problem when i started up my computer a few days ago. I got a yellow-ish orange-ish light on my tower and it wouldn't start up. All i had to do was take out the ram put the same ram back in and it worked. But this just may be something a lot more complicated or not complicated at all. It may be the processor or something else that is vital.
just give everything a good clean and apply some thermal paste. While you're getting rid of the dust and stuff make sure nothing is burnt out. If something is you'll have to replace it. But i'll give you a reply sometime tomorrow if you haven't fixed everything.