You have to simply diagnose your issue by unplugging all the wires (except keyboard, mouse, and monitor) and see if the computer will start up normally.
If that doesn't work then you will have to diagnose from the inside, start unplugging devices one by one (Cards, Drives, Battery, Ram, etc). Once the computer is working again, start placing hardware back in one by one. Toss or replace what was causing the problem.
If a problem still exists, try testing with a different power supply. If all still fails then it may be your motherboard. Damage or Overheating issues.