Logical:
Storage unit in the policy?

Physical:
Can robtest actually see the drives and slots - and actually move a piece from a slot into a drive, and back out of the drive and into its original slot?

Yes media moved to slot to drive and drive to slot. its working fine.

Describe your storage units?

Is this a new storage unit?  If so have you restarted the NBU services after creating the storage unit?

backup sucessful with storage unit few days back.now backup in queue.

Has anyone run the device configuration wizard recently?
Tinkered with manually deleting storage units and re-creating either manually in the GUI or manually from the command line?
Any zoning been performed recently?
Had any crashes or power loss incidents?

No sdw303.nothing will change.

I wasn't asking if anything will change.
We wanted to know if anything was changed around the time that it stopped working.

Master is v5.1.
Media is v4.5.

What O/S on the client?
What version of O/S (and any service packs or patches) on the client?
What version of NetBackup Client on the client?
What version of Oracle on the client?

Test to make sure that you can communicate both ways between the master and the media server.

Additionally run vmareq on the master to see that the media server is registered.  It should show the media server and the drives.

Next run vmoprcmd and look for the host in the list to make sure that it is showing "Active". 

If all else fails and it still fails to work, upgrade the media server, if it is a supported platform for 5.1. 
Also turn on all the logs to verbose level 5, remove all the current day logs, run the backup and wait.  If it still does not start, abort it and do a big log crawl so see if there are any of the little tidbits that may point to the problem. 

Since you mentioend it is an Oracle backup, have you verified that the DBAs did not do anything that would have dropped the Oracle RMAN and/or NetBackup modules from the DB as part of some upgrade?  Also what are the DBAs seeing when you kick off the backup?  Is the DB starting and backup and going into the proper mode for that backup?