When you cannot connect to the database, check the following:
1. Network situation. Check if tcp/ip is enabled and note the port number.
Open the SQL SERVER Configuration Manager and check the network configuration
2. Ping the server address to check for network problems.
3. Use the telnet command to check the port status.
If there is a problem with the network, check the firewall settings, or try turning it off first.
4. Check the server item. Whether the username and password are correct.
5. Whether the server allows remote connections, use Windows to verify the login and check to find Security -> Login Name -> Login User Name -> Right-click Properties, click Status:
6. Right-click on the server name -> Properties -> Security, enable two authentication modes
The above solves the problem of unable to connect to the database.