Resolving a Hung Database not able to login with sysdba

If database is not connected then use :

sqlplus -prelim  "/ as sysdba"

execute the oradebug hanganalyze command to analyze a hung database

SQL> oradebug hanganalyze 3
Statement processed.
SQL>

Oracle RAC environment, specify the oradebug hanganalyze command:

SQL> oradebug setinst all
SQL> oradebug -g def hanganalyze 3

Note: You can repeat the oradebug hanganalyze command a couple of times to generate dump files for varying process states.

You can get a systemstate dump from a non-RAC system by executing the following set of commands.

SQL> oradebug setmypid
Statement processed.
SQL> oradebug dump systemstate 266
Statement processed.

In RAC:

SQL> oradebug setmypid
SQL> oradebug unlimit
SQL> oradebug -g all dump systemstate 266