It could well be a hardware failure with your tape drive, so you might want to contact Dell support.
One thing to note from when I had the same problem with one of our installations, if you speak to Dell support they will undoubtfully tell you to install the latest firmware for the tape drive, which to me makes no sense since it was working before, however if they do it's worth pointing out to them that it will be impossible to do so since the installation of new firmware to the tape drive will require you remove any tapes from the drive before the installation will work.
I had to go through the whole process (requiring an onsite visit at a clients) just to find out I couldn't do what they were asking, so hopefully this will save you some hassle.