First of all, if your hard drive not recognized on your PC, make sure its connections are secure before moving further to other steps. Damaged USB cable, port problem, lack of power, or SATA connection issues are among factors which can make your computer unable to detect this hard drive. Try to reboot your computer and see if your hard drive appears in BIOS or Disk Management utility. In case if you have your hard drive in Disk Management, then try to find out the status of your hard drive and see if it requires any drive letter. You can also update or install storage drivers via Device Manager. Remember not to format or fix your drive until you are safe with your data.