There are many reasons which may causes external hard drive to show error message like not formatted. It could be Windows problem, i.e. hard drive itself does not have any problem, the Windows may have problem detecting the hard drive and hence show the error message. So you may try to restart Windows computer, or you may try the hard drive on other computer.
Sometimes the usb port may got problem, the connection may be loose, or the usb port may simply spoiled. So you may try another usb port and see whether the problem fixed.
You may also take a look at this article which explains how to fix the hard drive not formatted error and how to get files off the not formatted hard drive:
http://goo.gl/mIOJLp
Sometimes it could also be virus which destroys the file table, making operation system not able to read the hard drive well. If that's the case, you may run an anti-virus program to scan the external hard drive and try to kill the virus. Microsoft Security essential is a free and good one
http://goo.gl/rx1KFq
If all above failed, you may have to send the hard drive for repair.