The database service is not running: The database service is either not started or has been stopped, resulting in the inability to connect.
Incorrect username or password: The inputted username or password is incorrect, preventing authentication and connection to the database.
Firewall settings: The firewall or security settings have blocked the database connection.
Internet issue: unstable connection or network delay leads to connection failure.
Database listener failure: The database listener has malfunctioned or been improperly configured, resulting in the inability to connect.
The database instance is not running: The database instance is either not running or experiencing a malfunction, preventing connection.
The maximum number of conversations has been reached: The database has reached the maximum limit of sessions, and no new connections can be established.
Insufficient server resources: The server lacks the resources to support new database connections.
