My initial guess would be, there is something drawing too much current since the processor never seems to get to even try to boot the system.
Start by unplugging and/or disconnecting anything you can, one at a time. Like hard drives, removing ram, etx.
If you find nothing, remove the processor to see if it stays on. If so, it could be the processor or it may still be a MB problem.
Now you need to decide what you want to try to replace first.