Like Shorty said, I would start everything over. If you're using photoshop, go to image>adjustments>brightness/contrast and play with it until you think it looks nice compared to the background layer, then duplicate you're layer, press ctrl+u and put it all black, then place that layer behind the original, move it so it looks like you're shadow and change the opacity, for a better effect try messing around with Edit>transform>perpective, you could also put some lightning effects and some shadow in the layer style but I don't think it would look so good.