Oracle DB Shutdown issue Gave command: "SQL> shutdown immediate" to shutdown DB. But it is hung. 
Also I am not able to connect to DB via PL/SQL Developer. Its throwing error: "ORA 12537: TNS Connection closed
shutdown immediate will end idle sessions, but my understanding is that it will not kill of any statements which are in the middle of processing. But once issued, it will stop any new sessions from starting (thus you cannot connect any more).

So you can either wait for the operations to finish, or try shutdown abort.