Oracle 12c; cannot connect to HR without running ALTER PLUGGABLE DATABASE ALL OPEN
Check out this from Oracle-Base about starting and stopping pluggable DBs.

First, they provide a trigger to auto-start your PDB:

CREATE OR REPLACE TRIGGER open_pdbs 
  AFTER STARTUP ON DATABASE 
BEGIN 
   EXECUTE IMMEDIATE 'ALTER PLUGGABLE DATABASE ALL OPEN'; 
END open_pdbs;
/


And from Oracle 12.1.0.2 onward, we have this command to have Oracle restart the pluggable DB in whatever state it was in at that time (so open the PDBs then run this for each):

ALTER PLUGGABLE DATABASE pdb1 SAVE STATE;


But see the link for full details on both examples.