Is the drive a standalone drive? If so give these steps a shot.
From the Devices tab in Backup Exec.
1. Right click the tape drive, uncheck enabled.
2. Righ click ont he tape drive, select delete. You may get prompted to retarget some jobs, just restart them to "All Devices" for the time being.
3. Go to Tools - Backup Exec Services
4. Select Restart All Services.
5. Once it has completed restarting all the services, check the device again to see if it is still offline and that it is listed in Backup Exec.
Also check to make sure that the drive is listed in Device Manager, it should be listed under Tape Drives.
Thanks.