Why do I keep getting ORA-01033: ORACLE initialization or shutdown in progress when my database is open? (12c)
I can't tell you why this happens, but I get such errors regularly with the pluggable database. What I do as a quick fix is I connect using sqlplus "/ as sysdba", then I run the following code for that specific database:

alter pluggable database orclpdb open;


Or the following for all databases:

alter pluggable database all open;


And I stop getting the errors for a while... until the next time it happens. :)
At least I have a quick procedure as a fix.