If you get ORA-01033 error, it means that the Oracle instance you are accessing is being started or shutdown. Then, you cannot execute the action you are trying at the moment.
If you are the instance’s DBA and someone gets ORA-01033 error message without you submitted the start or shutdown commands, then you should verify the alert.log file and check for the reason of the shutdown. Depending on your installation (if you are running a RAC see our article on Split Brain in Oracle), you may also find useful information in other log/trace files.
Also check alert.log if you try to start the system but database users stills get ORA-01033 error for a considerable time (which also depends on your system/installation) after you submitted the startup command. This may mean an error occurred and the Oracle instance did not start properly.
You can also check the instance and database status in V$ views. Connect as sysdba and query for open mode of the database. Then, depending on the system’s current status you can also get other error messages (for instance, if the database is not mounted you can get ORA-01507):
Then, try to mount and open the database, as in the following figure. If you execute alter database open successfully, users should be able to access the database.
But do not forget to check out the log files and investigate why your database was shutted down!