Normal things that go wrong and you should check up on a “total black” screen:

• Try to reset via reseting power
• Are the fans running?
• Is the power supply working (exchange)?
• Is the ram connected (replug, exchange)?
• Is the cpu ok (exchange)?
• Is the graphics card working (replug, exchange)?
• Is the mainboard directly screwed to the frame (shortcircuted)?
• Is the mainboard screwed with too much pressure (cracks)?
• Unplug LEDs, Front USB etc. (maybe shortcircuted)
• CMOS Reset
• Try to plug out and remove everything not needed to show your some bios messages (even Harddrive and Cdrom)
• What about the internal speaker? Got some nice post beeps? (Check out manual of the vendor!)