Hi there Sim___,
That is really unpleasant.
🙁
Unfortunately, there is a chance that there is something wrong with the enclosure or the HDD itself. Yet, you can perform several simple troubleshooting steps.
I would be nice if you just start with attaching the drive with a different USB cable to another USB port. If you attach it to another system, you will be 100 % sure that the issue is caused by the device but not your system.
Do you have some data stored on the drive? In case the issue persists, you can try to access the drive with a data recovery tool:
http://www.tomshardware.co.uk/forum/id-1644496/lost-data-recovery.html
After that, you can just test it and see what is wrong with it...