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!)