That means the database was shutdown in the middle of your session, or the listener was shutdown so your SQL*PLUS session lost communication with the ORACLE database. When something like this happens, you have to reconnect.
Dig Deeper on Using Oracle PL-SQL
Related Q&A from Azim Fahmi
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.