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Have you checked on the load on the server at that time via top command if you are using Linux.
Also Did you find any error message in the alert log file of your database.
i used top command but there is nothing that is using cpu,
and there is no error in alertlog as well. It happened after every few days.
Have you checked on listener.log or some trace file generated in udump directories
Set sufficient value of processes/sessions in init/spfile of database , might be session limit of DB is getting exceeded & restricting new connection.
Check alert log & below query o/p , CURRENT_UTILIZATION < MAX_UTILIZATION to get connection to DB
Also , try to connect as sqlplus prelim & do hang analyze , refer below link
select * from v$resource_limit;
sqlplus -prelim "/as sysdba" ORADEBUG HANGANALYZE 3