Sqlplus Unable to connect to a DB instance workaround

1 See if Oracle's monitoring and Oracle's services are all started. To start Oracle snooping:
CMD in the command-line window, enter Lsnrctl start, and press ENTER to start monitoring.

2 to see what ORACLE's SID is called, such as when creating a database, the instance is called "abc", then manually set the Oralce sid,cmd Command window, set ORACLE_SID=ABC

3 re-enter Sqlplus  /nolog, return
Re-enter Conn/as sysdba;

4 re-enter startup, enter. This step is to start the Oracle service. If startup is notified that it has been started, you can enter shutdown immediate first, and then enter startup after the shutdown is finished.

5 After a few seconds, and so on, the command can be connected. At this time, you can enter "select * from User_tables;" Test to see if there are any query results.

6 The causes of ORA-01034 and ORA-27101 are manifold: Oracle's current service is not available, shared memory realm does not exist, because Oracle does not boot or does not start properly, Shared memory is not assigned to the current instance. Therefore, start the database by setting the instance name and then using the operating system authentication method. In this way, the database starts normally, and the ORA-01034 and ORA-27101 are not reported as two boot exceptions.

Sqlplus Unable to connect to a DB instance workaround