Reclaim the unused disk space

Hi,

  Database_name = OperationsManagerAC, database_size= 177251.44 MB, unallocated space =  76186.63 MB

From the above its been understand that there is unused space and how do I re-calim the space now, as we are totally running out the space even can't able to shrink the DB.

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Regards Hari Vidya Sankar