If an error occurs during catalog migration to Unicode, the system changes the operational status of the database to OFFLINE and you receive an error message. For information about the cause of the error, see the log file knldiag.err.
After an interrupted migration you only have the option of using the latest complete data backup to return to the status the database was in before the migration.
To do this, use the parameter file to re-create the database from the data backup, and then import the data from the backup into this database.
Database Administration, Log Files
Database Manager CLI Tutorial,
Copying a Parameter File from a Data Backup
Creating a Database Copy (Importing a Data Backup into Another Database)