If hard drive is connected to a desktop PC, please open up the protecting case and check the cables. Also, when the computer boots, go to BIOS (Boot Operating System) and check if the drive is detected. Also, boot your computer into Windows or what OS you're using and go to Device Manager/Computer Management/Disk Management. If the drive is identified, make sure to attach a letter to all its partitions. Contrary, no detection means that the HDD read & tag lines are broken and requires replacement. In this case, I am afraid your data is completely lost.