RE: XPS 8500 will not start, orange light on motherboard, will not boot, BIOS inaccessible, black screen
Orange LED on motherboard means absolutely nothing. It only indicates that 5VSB power is available.
Green and Yellow Lines indicate GPU fan failure and overheat damage.
Disconnect the Data Line for the hard drive and turn on.
It should show failure at that point.
If it does not there are other problems.