Please use following connection string syntax for connecting MSSQL DB from your ASP/.NET scripts.
This is a sample connection string which you can use with MS SQL 2005 / 2008 , ASP.NET 3.5 and above versions.
You need to create below connection string in web.config file.
<add name="ConnectionString" connectionString="Data Source=(database host);Initial Catalog=(database name);User ID=(database username);Password=(database password)" providerName="System.Data.SqlClient" />
Important Properties to change in connection string.
1) Data Source : This should be your MSSQL server name or server IP address like "localhost" or "X.X.X.X".
2) Initial Catalog: You can find database full name in plesk panel where you orginally created the database.
3) User Id : Replace with user name you created on top of MSSQL DB in Plesk control panel.
4) password: Password of the MSSQL DB user created in Plesk control panel.
Once you created connectionString in web.config file, you can use it in your ASP pages to retrive the database.
To read the connection string into your code, use the ConfigurationSettings class.
string connStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
Note: You can't use the same connectionString on your local PC to connect MSSQL DB server. It won't work as we don't allow remote connections to MSSQL server. It means you have to use/connect to MSSQL DB server through ASP/.NET scripts hosted on the same server.
Another option is to use the following string:
Data Source=localhost;Initial Catalog=Your_Database;Persist Security Info=True;User ID=Database_Username;Password=Database_Password
The "Data Source=" will depend in the database version you chose for the database in Plesk. You may need to use .\MSSQLSERVER2008 or .\MSSQLSERVER2012 instead.
MSSQL SQL Connection string