Well I can come up from two solutions. Im sorry nobody else replied to yours so im going to give it a try. First of all , were you recording on your HDD something?
If you record something on TV to your HDD it becomes locked and you cant access it via computer. Its anti piracy lock that uses any Smart TV today. Personal experience from SAMSUNG LED TV.
Second thing could be, that it is really broken, circuit board is not working properly or the HDD itself is gone. You can try to take it apart and try HDD to mount it into computer. Talking about big case pc. But I wouldn't recommend that since by taking apart the the external case would void your warranty.
If nobody responds this , im afraid that the only way to make your HDD work again is taking it to seller and let him replace for the new one. Ultimately you can try to connect it to tv and if you can format it through tv -> do it. You would lose all your movies but you would spare yourself a trouble with seller. If that doesnt work/help either... Seller is the way of life...
EDIT: If your warranty is over take it apart and mount the HDD to the computer and see if it is working.
😉
EDIT 2: If it works properly in media player it seems that the media player itself locked it for antipiracy issues.