I presume you are using a 7 pin connector to the drive and are certain it is connected properly. Sometimes they can be a little fiddly as can the data connector; this latter should give a small click as it engages.
When you replaced the drive, did you connect to the same point on the motherboard as the original drive was connected to?
When you turn the PC on, can you hear the drive running?
You may also get a BIOS screen which tells you that drives have been detected; do you see this?