Check and make sure that if it's showing under 'Disk Management'. Go to My Computer > Right click and choose Manage > Choose device manager from left side menus. If it does, consider formatting it to start everything from scratch.
Alternatively, you can also try connecting it to different device and see if the same happens again. Sorry to say but I strongly suspect that it may started failing or soon become completely damaged.
Try using recovery software like Stellar Phoenix or Recuva if you don't want to lose the stored files.