Query on Container-Pluggable database
If your pluggable database is not open, then you will not be able to connect to it in regular fashion. You need to check the connection descriptor that you application uses in order to check where it connects.

When you create a PDB, it is automatically added in the listener as a service. You can check using

lsnrctl status


To connect to a specific pluggable database, you need to use its service name.
You can start a pluggable database by connecting to the container and issuing:

alter pluggable database &lt;PDB_NAME&gt; open;


To switch your context to a specific pluggable database when connected to the container, use:

alter session set container = &lt;PDB_NAME&gt;;


After that all commands will be ran against that pluggable database.

References:

Introduction to the Multitenant Architecture

Connecting to Container Databases (CDB) and Pluggable Databases (PDB)