FAILOVER in dataguard

SQL> Select name,open_mode,database_role from v$database;

NAME      OPEN_MODE            DATABASE_ROLE
--------- -------------------- ----------------
PRIM      MOUNTED              PHYSICAL STANDBY

SQL> alter database recover managed standby database cancel;

Database altered.

SQL> alter database recover managed standby database finish;

Database altered.

SQL> alter database activate standby database;

Database altered.

SQL> Select name,open_mode,database_role from v$database;

NAME      OPEN_MODE            DATABASE_ROLE
--------- -------------------- ----------------
PRIM      MOUNTED              PRIMARY

SQL> shut immediate;
ORA-01109: database not open


Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  780140544 bytes
Fixed Size                  2929256 bytes
Variable Size             557845912 bytes
Database Buffers          213909504 bytes
Redo Buffers                5455872 bytes
Database mounted.
Database opened.
SQL> select name,open_mode,database_role from v$database;

NAME      OPEN_MODE            DATABASE_ROLE
--------- -------------------- ----------------
PRIM      READ WRITE           PRIMARY

You might need to recreate standby from current primary.