If you are getting the following errors when trying to connect to your Oracle database.

ORA-01034 ORACLE NOT AVAILABLE
ORA-27101 SHARED MEMORY REALM DOES NOT EXIST
Checklists to Check:

-Check your TNANAMES.ORA and SQLNET.ORA files and make sure it is correct and pointing to the correct database.

-Check your Server’s logs to see if their are any events logged.

-Check your disk space and memory, and make sure it isn’t used up.

-If everything looks normal, restart the server and see if you can get the Oracle database back up.