leeahye2

Do a simple step before you apply data recovery program to it:
1) It is worth trying to attach it to a different USB port, try one on the motherboard back plate.
2) Go to Command Prompt and do a CHKDSK/f. There are many ways to access Command Prompt:
If you are under Windows operating system, go to Start > All Programs > Accessories, right-click on Command Prompt and select Run as administrator. In CMD window that appears type this:
chkdsk x: /f
(where x = drive letter associated with your external hard drive, and before x: and after x: there is a space)
and press Enter.
Command chkdsk (check disk) will correct errors and your external hard drive may be accessible again.
Good luck. If failed, You may be able to recover files from the drive with  Recuva, PhotoRec, iCare Data Recovery Free, Easeus Data Recovery Wizard Free, etc or other recovery program.
After you recovered all your files to the healthy hard disk, then format the external hard disk and see whether it can work again or not.