1. Insert the last successfully written full tape in the span (it should have a status of "full"). It will generally be 1 tape back before the last that was tried.
Eg. If you were on tape 10 and the job stopped/failed on that tape, Tape 9 is likely the last successful tape.
If there was not a successful tape that was written during the job, you will need to start the job from the beginning.
If you plan to run a differential backup in the future of this same data, you will need to start the job from the beginning as the job needs to complete successfully in full.
2. Begin the full backup exactly as it was initially done. You're essentially attempting to backup the same data to this full tape.
There's a bit of leeway here but if the backup is 10% or more different than the original then it won't restart.
3. The job will resume and will take a bit of time to verify which files have already made it to tape and which need to be rewritten. The job will then ask for the next tape.
4. Insert a new empty tape (or one that can be formatted).