When a current redo log is lost, the transactions in the log file are also lost before making to archived logs. Since a DB startup can no more perform a crash recovery (since all the now-available online log files are not sufficient to startup the DB in consistent state), an incomplete media recovery is the only option.

We will need to restore the DB from a previous backup and restore to the point just before the lost redo log file. The DB will need to be opened in RESETLOGS mode. There is some transaction loss in this scenario.

Step 1: Restore Database (All control file, log file and data file) from Cold Backup
Step 2: Startup database with mount option.

SQL> startup mount

Step 3: Recover database using backup control file option.

SQL> recover database using backup controlfile;

Step 4: Cancel the recovery.

SQL> recover database using backup controlfile until cancel;

Step 5: Open the database with resetlog option: