I have moved your comment in this 7-year-old post to a new discussion : https://vox.veritas.com/t5/NetBackup/Robotic-library-is-down/m-p/459630
There were lots of suggestions in that discussion - have you read through all of them?
Have you verified that the OS can (still) see the tape library?
Have you checked output of 'scan' command to confirm that the library and tape drives are responding to scsi-commands?
Any device-related errors in Windows Event Viewer?
It seems that you've been having device issues for 3 weeks now, right?
Your screenshot shows that the OS only sees one tape drive.
Do you see the library in Device Manager under 'Medium Changer'?
As an application, NetBackup has no direct access to a device, instead relying on the operating system (OS) to handle any communication with the device.
Any troubleshooting should therefore start at the OS level.
As per the TN that I shared in your other post: