Sql server error: provider: Named Pipes Provider, error: 0 - No process is on the other end of the pipe

I have written a windows form application with C#.Net using SMO for backup and restore the database.  here is my connection string:
<connectionStrings> <add name="PhoneBookEntities" connectionString="metadata=res://*/Model.csdl|res://*/Model.ssdl|res://*/Model.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=.\SQLEXPRESS;attachdbfilename=|DataDirectory|\PhoneBook.mdf;Pooling=true;Min Pool Size=1;integrated security=True ...