Pull out the mobo and connect just one RAM stick, a keyboard and your graphics card and see what your motherboard does then. If it still doesn't boot then it's definitely one of those pieces. My computer wouldn't boot, it'd power up but nothing, no post no nothing, turned out, when I had seated my ram a hair had fallin across the slot the RAM falls into so I pushed the hair in with the RAM.....shorted two circuits together and computer wouldn't boot. As soon as I pulled the hair out and reseated the RAM it started right up.
Don't suppose you have a digital camera? Then you could take a picture of this 'scratch' if it did indeed go across a circuit then yeah it's very likely that could have caused some damage, but we'd really need to see where it is and how bad it is to really tell. Also there ARE ways to repair that but it takes patience, a steady hand, and the will to do it I suppose. Usually need conductive paint and a razor blade.