Hi @Xiaowen Dong
As Olaf says, you can enter your server name directly. If you don't know your server name, you can use the statement "select @@servername" in SSMS.
But in general, when you create a connection string, clicking the drop-down will automatically pop up the connectable server name. I guess you mean that? You can check your Sql server configuration manager. Make sure that everything except the sql server agent is starting.
Best regards.
Aniya