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Recovery depends on whether the tenant that applied the Purview/RMS protection still exists and has an administrator available.
If you can identify the original tenant, a Global Administrator or Purview Administrator may be able to assist. If the tenant has been retired or is no longer accessible, recovery may not be possible because the encryption keys remain with that tenant.
I recommend opening a Microsoft Support case and providing the former account details and any information you have about the tenant. They may be able to help identify the tenant owner or advise on available recovery options.