In the logs, trace or in output from a Media Manager check the location of the backup index may be given.
/usr/netvault/db/MediaDatabase/Files/Backup/0001/9e/84
If the index file is not there, it is "Offline". NetVault Backup may know about the existence of the backup, but the index was removed. If this is the case, you can right click on the backup in the restore window and select "Load Index" to bring it back online, but to find the backup in the restore window you need to know the Saveset #.
The file path is actually derived from the saveset number in hexadecimal format. Using the example above you can convert 19E84 from hexadecimal to decimal and get 106116.