juda,
Without knowing your environment it is tough to answer this but...
I would first scan your server activity log for any possible errors related to the volume in question. Something must have happened to make the volume unavailable. Scan for I/O errors. Possible read/write error. If so, then I would run a audit volume with the default values to inspect the volume for possible damaged objects. If there are and you have a copy of the volume then you can repair and try your restore again. If you don't have a copy of the volume and there is damage and "active" files are damaged, you are in trouble.
Or... it could be as simple as the volume was checked out for some reason and just needs to be checked in. Do a q libvol and see if the volume shows up. If so then you know it is in your tape library (assuming you have a tape library)
Mike B
