If your primary database is not accessible or crashed for any issue, then we can use open the standby database for use.

Below are the steps for opening standby database when the primary database is lost:

 
1. start the standby database in mount stage:[ STANDBY]

2. Check the protection_mode and database_role

NOTE: If protection_mode is other than maximum performance, then alter it as below.

SQL> ALTER DATABASE SET STANDBY DATABASE TO MAXIMIZE PERFORMANCE;

3. Recover standby database:[ STANDBY]

Make sure all archives has been applied in this process.

4. Finish the recovery process:

5. Activate the standby database:

6. Check the status:

7. Open the database:

Now application can start using this database.