Hey there, sebk100.
Have you had any issues with the drive prior to this problem? Any unusual sounds/noises coming from it, low read/write speeds, hanging or freezing your system, etc?
Try the drive with a different computer and a different USB cable (preferably shorter than 12"), to see if the same thing happens. What does it say about the drive and its partition in Disk Management (you can post a screenshot and use imgur.com to upload the image)? Here's how to access it:
How to access Disk Management in Windows to manage hard drives.
Other than that, you could try accessing the drive via Ubuntu Live USB, to see if the HDD is properly recognized and if you can backup your data. Another option would be to go for data recovery...