I assume that your CDB$ROOT is open.
So, did you try to resize your CDB$ROOT (by shrinking one more datafiles)?
This could help you to come below the limit so that you can open your PDB.
I think the first things you want to do is to check the actual alert.log, which should be located at:
Check for clues.
Next, before you make any changes, I suggest to perform a full RMAN backup.
Perhaps you can reclaim some space from the temp datafile, though I'm not sure if the space counts toward the 12GB limit. But perhaps worth a try:
SQL> ALTER TABLESPACE TEMP SHRINK SPACE;