oracle error- ORA-00393: log string of thread string is needed for recovery of offline datafiles

Oracle error

ORA-00393:	log string of thread string is needed for recovery of offline datafiles
Cause:	Log cannot be cleared because the redo in it is needed to recover offline datafiles. It has not been archived so there is no other copy available. If the log is cleared the tablespaces containing the files will have to be dropped.
Action:	Archive the log then repeat the clear command. If archiving is not possible, and dropping the tablespaces is acceptible, then add the clause UNRECOVERABLE DATAFILE at the end of the clear command.