So I checked RDS and dashboard and found out that number of database connection was beyond allowed limits. It means connection open to RDS was not closed after sometime and there were lots of half opened connections. That eventually crashing RDS sever and then connection was not established till we restart the RDS server.
So next step was to check database queries and find the queries which are taking time to execute and get data. After looking into all the queries I got the query which is taking long time to execute and that query was responsible half closed or non closed database connections and which is ultimately crashing Amazon RDS server.
So how I fixed that. Well we have a only one solution to speed up database queries and that is