Check the System and Application logs for clues.
I would stop all Backup Exec services, and then take the library offline and use the front panel to remove the stuck tape. Take the stuck tape out and place it aside. I would then shut down the Backup Exec server and reboot the library. Verify the library comes up in an online mode. Then restart the backup exec server.
Sometimes when a tape gets stuck, the communciations between Backup Exec server and the library just hang, and there is nothing you can do. Shutting down the BE services should allow you to locally communicate with the library and remove the tape. Rebooting the library with the Backup exec server down proves the library is healthy and passes POST. After that it's time to start the backup exec server and check the logs for erors and and verify the device is properly detected in device manager, has drivers loaded, and try the inventory again.