Have the tapes aged? where the icon looks a bit like this? 

Or are they showing another status? (there is a full list here: Identifying Media Icons (commvault.com) ) If they have aged it means that the retention has expired, but there is a method to catalog tapes that have expired to create an index that you can then look at to pick a job (or jobs) to merge back into the Commvault database so you can recover.  The full process is listed here in documentation: Cataloging a Tape (commvault.com)
 

The jobs will likely have aged out.  You can review the documentation on Recovering Aged Data to learn how to catalog the data.  Be careful to not leave the tape in the library while you review the process to ensure they are not overwritten.

@tclawre I would first of all make sure the switch on the media itself is set to the overwrite protect mode. As @Steven R already said, as long as they are not being picked for new jobs they still contain all previous jobs so via the Recovering Aged Data option you can still recover. Do make sure to check your retention settings afterwards and if you are uncertain if the configuration is set correctly then I would recommend you to or open a post on the community or ask your Commvault representatives for assistance. 

Thank you all for the quick responses.

I would write protect the tape before importing into the library to prevent (as stated above) CV from trying to use it for a new backup.