There may be a problem with one of the drives; often if there is an excessive load (or not enough), the PSU won't power on.
For minimum configuration, you need only the RAM and a video card.
If you don't have a keyboard hooked up, the computer should beep at you (you might have to plug a speaker into the mainboard if it doesn't have a built-in one), the same if there isn't a monitor hooked up.
If that doesn't happen, then you might have a short from the mainboard to chassis, or there is an internal problem on the mainboard.
For minimum configuration, you need only the RAM and a video card.
If you don't have a keyboard hooked up, the computer should beep at you (you might have to plug a speaker into the mainboard if it doesn't have a built-in one), the same if there isn't a monitor hooked up.
If that doesn't happen, then you might have a short from the mainboard to chassis, or there is an internal problem on the mainboard.