To Resolve connected to idle instance

1. Connect the DB sqlplus / as sysdba

2. startup nomount

3. alter database mount;

4. Clear an unarchived redo logs

alter database clear unarchived logfile group 1;

alter database clear unarchived logfile group 2;

alter database clear unarchived logfile group 3;

5. shutdown immediate

6.startup

alter session set container=pdbdbname;

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