When accessing Oracle with SYSDBA, the following information is prompted:

[oracle@localhost ~]$ sqlplus/as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Thu Dec 2 20:21:40 2021
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> select * from dual;
select * from dual
*
ERROR at line 1:
ORA-01034: ORACLE not available
Process ID: 0
Session ID: 0 Serial number: 0

solve:

First, make sure to start listening:

[oracle@localhost ~]$ lsnrctl start

Then start instance:

SQL> startup;

This is the open status when you view the database instance again:

SQL> select status from v$instance;

STATUS
------------------------
OPEN