CS0246: The Type Or Namespace Name 'SqlConnection' Could Not Be Found (are You Missing A Using Directive Or An Assembly Reference?)

Hello,I just lost all my data doing a system restore and now trying to rebuild my web project.However, Am getting the error below, trying to use a class to run my Stored procedure.What is wrong herethanksEhiCompiler Error Message: CS0246: The type or namespace name 'SqlConnection' could not be found (are you missing a using directive or an assembly reference?)

Source Error:

Line 22: //SqlConnection con = new SqlConnection("cellulant_ConnectionString");
Line 23: //SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["cellulant_ConnectionString"].ToString());
Line 24: SqlConnection con = new SqlConnection(ConfigurationManager.AppSettings["cellulant_ConnectionString"].ToString());
Line 25:
Line 26:
Source File: c:inetpubwwwrootcellulant1App_Codesignup_data-entry.cs    Line: 24
  using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;


public class signup_data_entry
{
public signup_data_entry(DateTime dob1)
{

//SqlConnection con = new SqlConnection("cellulant_ConnectionString");
//SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["cellulant_ConnectionString"].ToString());
SqlConnection con = new SqlConnection(ConfigurationManager.AppSettings["cellulant_ConnectionString"].ToString());


SqlCommand command = new SqlCommand("Cellulant_Users_registration", con);
command.CommandType = CommandType.StoredProcedure;

con.Open();