hi. did your hard disk open before or it's the first time to use it?
usually it a virus affected the hard disk that makes your hard drives disappear or not open.
try this remove it and then plug it in then restart. if it's not appearing use this start > control panel > performance and maintenance > administrative tools > computer management > disk management > choose your volume letter ( example F: or G: or any letter) choose the box with stripe or colored line then left click scroll down to properties then choose autoplay page then check or select action to perform choose open folder to view files then apply and ok.
it should appear just fine, but if the problem keeps up i suggest that after this just copy the files you want and format it other wise this might happen again some other time and you lose your files.