oracle error- ORA-00030: User session ID does not exist.

ORA-00030: User session ID does not exist.

Cause:     The user session ID no longer exists, probably because the session was logged out.

Action:     Use a valid session ID.

oracle error- ORA-00029: session is not a user session

ORA-00029:  session is not a user session

Cause:     The session ID specified in an ALTER SYSTEM KILL SESSION command was not a user
session (for example, recursive, etc.).

Action:     Retry with a user session ID.

oracle error- ORA-00028: your session has been killed

ORA-00028:  your session has been killed

Cause:     A privileged user has killed your  session and you are no longer logged on to the 

database.

Action:     Login again if you wish to  continue working.