Hi,
For restoring the data from the tape you just need to inventory the tape first so that Backup Exec reocgnises the tape and then "catalog" the tape.
If the "Catalog" job is successful then the data on the tape can be restored..
As far as the tape becoming "unusable" is concerned , it depends on the hardware being used and the way its stored on an off-site location...
Please note that if Backup Exec fails to recognise the tape due to its "unusable" condition it may not be possible to restore the data from the tape.
Regards
Padmaja