It shouldn't make a difference since it was working before, but try changing the pin setting on the back of the drive just in case.  For example, if the pins are manually set to primary and slave, then try changing both drive to cable select.  If it's already set to cable select, then try changing one to primary and the other one to slave.

Or, if you have access to another system nearby, try connecting the drive to that.  Conversely, if you have a known working drive, then connect that to your system using the ide channel which was used by the drive you just replaced.  All you are trying to do here is narrow down the possible causes as much as you can to see if it's the drives that actually failed.

If the drive still fails to recognize on another system, then unplug the drive and let it sit idle for at least a day before giving it another try.

Hope that helps.