|If a drive is not recognized, it can be only:|
1- The hard drive is bad
2- The power is not getting to the hard drive (power supply or cable/connector)
3- The signal is not getting to the motherboard (bad signal cable)
4- Bad motherboard/SATA controller
(Possible other problems too rare to mention at this point)
Begin by eliminating them one at a time. Try a different power connector. Try a different SATA signal cable. Look for a screw you may have dropped when removing the old hard drive that might be causing a short under the motherboard near/under the SATA connector block. Try a different power supply if you have one around (very possible).
You have to be a little bit crazy to keep you from going insane.