|One thing that can raise it's ugly head if a computer is left unused for years is oxide on the edge connectors - often barely visible. It would be worth pulling out connectors and re-plugging then a few times. Also use a soft pencil eraser on all edge connectors (RAM, SATA Power & Signal, add-on cards including graphics if appropriate).|
Capacitors have a knack of requiring reforming so leaving the computer on for a few hours might help too.
Always pop back and let us know the outcome - thanks