Type command WRKPRB on your AS/400 check all the MSGS . DID you try to Vary off and Vary ON your Tape device . Check the power on you LIB is Diplay panel is showing any error . I faced similar problem it was drive power failure .
If this doesnot work then your tape device is failed This error come when DRIVE 1 on tape lib failed . Check the MSGS and call your IBM HARDWARE SUPPORT. Maybe you might have to change LIB or you have to deleted you tape device from WRKDEVD and create them again .
Better call IBM Support soon .