On the My Computer icon in Windows,
right click, select Manage, click Device Manager. Do you see your tape device there?
If not, you'll need to run Add/Remove Hardware.
If it is there, try stopping and restarting Backup Exec services (all of them).
If none of that works....is the tape device a part of the server or is it a library? Part of the server, just reboot the server. A library, reboot the library, then reboot the server.
-SQLBill