Hey Paul. This does not sound very good indeed. I'd recommend that you try the drive with another computer and a different USB cable (not longer than 12"). Hopefully the cable has become faulty or something. Do you have any important data on it? If "yes" I'd recommend that you backup everything if you can still access it. If not you could try taking the drive out of the enclosure and connecting it directly to the motherboard to get your data out, but note that this will probably void the warranty of the drive. This might work if the problem is due to it's external enclosure.
It would also be helpful if you could download the manufacturer's diagnostic tool and test the drive with it, but not before you recover any that which you might need from it.
Hope that helps. Let me know if you have any questions.
Boogieman_WD