May be the device file is corrupted.
If it a TZ tape try recreating the device file. It it is the first tape drive, first remove all /dev/nrmt0*. Then in /dev directory do this command
./MAKEDEV tz*
If the TZ tape is connected and detected, then the associated device files will be created such as /dev/rmt0a /dev/rmt0b ... /dev/rmt0h, /dev/nrmt0a, /dev/nrmt0b
If this doest not work, please post the output of command "show dev" from >>> prompt. Good Luck