How to restore an offline media and move it back to library slot?

I've just finished setting up a backup server. When testing the server,  i'm having issue with one of these tapes which I've originally put into slot 6. The job that uses that specific slot failed becuase of the media being as offline. I cancelled the job and now slot 6 is showing as empty and that tape just sits in the offline category. I've tried cataloging the media but the job keeps queuing and has done nothing. Can anybody tell me how to change the status of the tape from offline to online and get it moved back to slot 6 as it was originally set up.