Open yours portal log and look to see if the DB connections number is available.
For Oracle DB you can see message like below:
ORA-12516, TNS:listener could not find available handler with matching protocol stack
Increase the number of connections on the DB size.
ALTER SYSTEM SET PROCESSES = 500 SCOPE=SPFILE;
ALTER SYSTEM SET SESSIONS = 555 SCOPE=SPFILE; -- 1.1 * PROCESSES +5
Make sure how many connections yours DB has and increase for yours needs.