Hi,
I am sorry to hear that you are facing this issue with the external hard drive. I will try and help you get the issue resolved; however before we begin could you provide us some additional information.
1. Do you have any anti-virus program installed on the computer?
2. Have you tried connecting the drive to another computer and checked?
3. Are you aware of any recent software or hardware change on the computer?
You may check if the drive is being detected in Device Manger. If it does, right click on the device and click on uninstall. Restart the computer and let it find and install the drivers automatically. Check if this helps. If the issue persists you may assign a drive letter and check if it helps. Follow the steps from the article and check.
Hope this information is helpful and do let us know if you need further assistance. We will be glad to help.