The tape might be frozen due to any issue which occurred when it was being used the last time - i.e. due to drive, robot or tape drive having issues (broken leader, damage, etc). If you are certain that there are no physical issues with the tape cartridge and tape leader is intact, the "procedure" is the usual one:
execute bpimagelist -d 01/01/1970 -l | grep XXXXXX
where XXXXXX is your tape media ID, if no fragments are associated with that tape you should unfreeze the tape and NBU will place it back into scratch. If fragments are found, you can identify which COPY number they are and then search of the backup ID´s using the COPY # as the might not be primary copy anymore.