have you tried new IDE cables, cause it doesn't sound like your HD, plus, I would try to go into the BIOS and redetect the HD, that could possible work, but otherwise, idk?
Also, if you are having trouble turning on your comp, make sure you didn't loosen the power cable that run form the turn on switch to the mobo, they ALL have to be connected except the HD light and the power light to work, double check that.