depends on the damage , in the cylinder that dropped the valve the piston will be damaged and more than likely the combustion in the head will have got a bit awell,if it stopped the engine when it dropped its also possible that it bent a con rod,its more than likely that the oil light came on because the engine had stopped.but you will have to remove the head to get an idea of were to go next.bring it to a good specialist and they will give you a proper assesment of the damage but sometimes you can be lucky and get away with a piston and valve but not that often