If the database is brought down and up again while pl/sql developper is connected, the command window works but the sql*Windows give ORA-3114: not connected to ORACLE error.
If the server was shutdown this can indeed happen. You will need to logoff and logon again. If the "Check Connection" preference is enabled (Tools > Preference > Options tab page), the logoff will automatically occur.
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Marco Kalter
Allround Automations