I observed one more thing while trying to fix this.

I logged in to database(in the node which was not shut down), using command :
conn sys as sysdba.

Then it connected to idle instance.

Then I used the command startup, which then started the oracle instance. After that I could connet to database from outside as sysdba only. For other users, it's showing oracle initialization or shutdown in progress. That was a bit strange though.

So does that mean, Out of my two nodes, if one node is shutdown, it's not starting oracle instance by default ?

And if so, how to fix this.