With the external HD plugged in, find it in the My Computer window, usually listed under removable storage devices. Right click on the drive icon and in the pop-up menu click on "properties". A window about the drive's properties should open. Click on the "Tools" tab at the top of the window, and then click on the "error-checking" button. Another small window will open. Check both the boxes in the window and click on "start".

Windows will now run the Disc check utility on the drive and should fix any file errors found. Once complete, try and see if you can access it now.

If this didn't work, there are applications available that can scan and recover lost or accidentally deleted data on hard drives an external devices. One I can recommend is "System Mechanic".