Clearing the redo log files is usually the first remedy used in
repairing a damaged instance ; it's the equivalent of dropping and
re-adding each file.
SELECT * FROM v$log ;
ALTER DATABASE CLEAR LOGFILE 'filename' ;
/* if archive log mode is true, may need to force the clear */
ALTER DATABASE CLEAR UNARCHIVED LOGFILE 'filename' ;
ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 2 ;