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

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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.