Just follow standard HDD troubleshooting procedures. Start by putting everything back as it was before the problem occurred. If the drive spins up but isn't detected by the system, then obviously something has failed or changed. Note that Dell normally used cables supporting "Cable Select", so make sure the jumper on the drive is set for CS instead of Master or Slave. With CS cables the blue connector attaches to the motherboard, the black connector at the other end attaches to the primary drive and the middle grey connector attaches to the secondary drive (if used). If the system still isn't detecting the drive when it is properly installed, then either the cable, drive, or motherboard are defective. Try replacing the cable and drive with known-good spares, if that fails but the rest of the system appears to be working, rather than replacing the motherboard I would just install a new PCI to Parallel ATA (IDE) adapter
. Let me know what you find, Scott.