Backup Exec 2012 problem

Dear All,

                  In my environment Symantec Backup Exec 2012 is running on windows server 2008 at friday evening  i was scheduled daily differentials jobs when i came to office and see the backup jobs are not running and is shows that    "Ready; The selected server is not a valid managed Backup Exec server.  for this problem resolution i have done the below mention things.

                  1. restart the server

                  2. restart the service of Back up Exec 2012

         but still same result waht should i do to resolve this problem any suggestions.

thanks

Fasih