Please post ALL of your replies here, so anyone who reads this thread has the opportunity to learn what steps you've taken to solve your problem.
If you have stripped the system down to the bare essentials but you still have no signal on the monitor, shut the system down again, UNPLUG IT, then remove and reinstall the graphics card, unless your system uses onboard video. Be SURE to DISCONNECT all of the hardware I mentioned in my first reply; DISCONNECT the power AND data cables from the HDD, the optical drive, any cards you might have installed, any external devices connected to a USB or FireWire port, EVERYTHING except the bare essentials. Once you are working with the basic system ONLY, try to boot the system again and let us know what happens. If the system still shows nothing on the monitor, try using a different monitor.
Testing this system will probably require a process-of-elimination method; swap ONE COMPONENT at a time (monitor, graphics card, motherboard, etc.) until the system works again or until you've determined the cause of the failure.