When you did all this you could have messed up the bios if u took power off is ur battery low ? is it a slave or master, if you had to take the ribbion off to get at the ram slot's is back in right order ???
All I can think is that you have dislodged the drive cable while you inside the case. Are you sure the hard drive has failed. ie does the machine simply fail to boot? Do you get any messages? How far does the boot sequence get, does the memory test succeed?