What is the reason that the SQL server cannot connect

Database connection failure may be caused by the following reasons:

1. Server failure: The database server may fail or be down. You can try to restart the database server or contact the database administrator to solve the problem.
2. Network problem: There may be a problem with the network connection between the database server and the client. You can check whether the network connection is normal, such as network configuration and firewall settings. You can also try other network connection methods.
3. Configuration error: When the client connects to the database, it may use incorrect connection information, such as the host address, port number, user name, password, etc. of the database. You can check whether the connection information is correct or try to reconfigure the connection.
4. Database permission problem: When the client connects to the database, it may lack the necessary permissions. You can check whether the user name and password are correct, and whether you have permission to connect to the database.
5. Database overload: The database server may be unable to respond to new connection requests due to processing too many query requests, resulting in connection failure. You can try to connect again after waiting for a period of time, or adjust the frequency and load of query requests.
6. Other problems: There are other possible problems, such as the database server is not started, the database file is damaged, and the database connection pool is full. You can check the log file of the database server or contact the database administrator to view detailed error information.