ORA-01157:cannot identify/lock data file string - see DBWR trace file

ORA-01157:cannot identify/lock data file string - see DBWR trace file
Cause: The background process was either unable to find one of the data files or failed to lock it because the file was already in use. The database will prohibit access to this file but other files will be unaffected. However the first instance to open the database will need to access all online data files. Accompanying error from the operating system describes why the file could not be identified.
Action: Have operating system make file available to database. Then either open the database or do ALTER SYSTEM CHECK DATAFILES;