[WebMethod]

public string hello()
{

string sConn = ConfigurationManager.ConnectionStrings["travel"].ToString();

string sSQL = "select *from tbl_login";

SqlConnection connCustomer = new SqlConnection(sConn);

DataSet dsCustomer = new DataSet();

SqlDataAdapter daCustomer = new SqlDataAdapter(sSQL, sConn);
daCustomer.Fill(dsCustomer,"Customers");

return "hello"; }

You need to return some string from the function. Check the return statement.