You won't be able to run any tabadmin commands to perform a backup or interface with the UI to download any workbooks manually until the licensing issue is resolved. Are you transferring environment or need assistance with data migration? A case with Tableau Technical Support is likely necessary to resolve the licensing issue.
Suddenly we got this issue.
we are not doing any environment migration,as we thought this issue is happened so we thought of taking all the workbooks and keep it in a backup file.
I would highly suggest a case with Tableau Technical Support (http://www.tableau.com/support/request). The Tableau Server becoming unlicensed unexpectedly can indicate an issue with permissions, virtual machine configuration, disk corruption or a host of other issues. Tech Support can investigate your product key and determine how it became unlicensed and work to quickly resolve the issue. If this is your production environment, the case would be given a high-priority and worked quickly - likely within a few hours.
There are too many factors and we do not have access to the tools in the Tableau Community to help with a licensing issues - tech support is a much better resource in this instance.
Thanks and hope you're able to get help from Tech Support!