Computer will not boot up - black screen

Hello,
My computer will not boot up. When I turn it on, the fans begin working as if the computer is loading, but the screen remains black and nothing loads. Shortly after attempting to boot up the computer, it then tries to restart itself, either turning off completely or attempting to boot again. I have tried switching video cards, removing the ram, re-wiring everything, cleaning dust out, switching monitors... And nothing has worked. The computer parts are about 2 years old, the motherboard and the power supply being the newest parts currently. Everything lights up, fans work on everything, no beeping or abnormal sounds, but there is still nothing on the screen whatsoever. Previous to this issue, my computer was randomly shutting itself off without any warning, sometimes when it was in use and sometimes when nothing was being done to it. Any help or suggestions would be greatly appreciated.