Try plugging into another USB port.
Plug in the external HD, type Disk Management into start search box, rightclick to Run as Administrator.
In Disk Mgmt, look for the external HD listing to see that it has a letter. If not give it one: Drive Letter - Add, Change, or Remove in Windows 7 - Windows 7 Forums
If it has a letter, then click Start>Computer, rightclick on the External to run Disk Check.
If HD is not detected or will not run Disk Check, install and run free Partition Wizard
which may see the HD, give it a letter if not one, then rightclick on it to Check File System.