Go to the Devices tab, right-click on the tape library/drive and pause/unpause it.
Also, check that the tape library/drive is functioning correctly in the Windows Device Manager.
If these do not work, then
1) Delete the library from BE
2) Stop the media server and the library
3) Start the tape library
4) Start the media server
5) Ensure that the library and tape drive is recognised and functioning correctly in the Windows Device Manager. The library should appear as an Unknown Medium Changer with a Microsoft driver and the tape drive should have a Symantec driver.
6) In BE, install the library if necessary. Go to Tools --> Wizards ---> Device Configuration Wizard to do so.