Also unplug everything from the back except for the VGA cable and see it if comes on. I can't tell you the number of hours that has been lost in the shop because of a keyboard or mouse that was shorting everything out.
That's the first step. Because it could be.....
1. Power supply
2. Memory
3. CPU
4. Short in a keyboard
5. Short in a mouse
Also unplug everything from the back except for the VGA cable and see it if comes on. I can't tell you the number of hours that has been lost in the shop because of a keyboard or mouse that was shorting everything out.