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 will be accessible again.