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]

shut immediate;
startup mount;

2. Check the protection_mode and database_role

SQL> SELECT OPEN_MODE,PROTECTION_MODE,DATABASE_ROLE FROM V$DATABASE;
OPEN_MODE     PROTECTION_MODE            DATABASE_ROLE
----------   --------------------      ----------------
MOUNTED       MAXIMUM PERFORMANCE      PHYSICAL STANDBY

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]

SQL> RECOVER STANDBY DATABASE;

Make sure all archives has been applied in this process.

4. Finish the recovery process:

SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE FINISH;
Database altered

5. Activate the standby database:

SQL> ALTER DATABASE ACTIVATE PHYSICAL STANDBY DATABASE;
Database altered.

6. Check the status:

SQL> SELECT OPEN_MODE,PROTECTION_MODE,DATABASE_ROLE FROM V$DATABASE;
OPEN_MODE     PROTECTION_MODE            DATABASE_ROLE
----------   --------------------      ----------------
MOUNTED       MAXIMUM PERFORMANCE        PRIMARY

7. Open the database:

SQL> ALTER DATABASE OPEN;

Now application can start using this database.