Help in the login

Question

I have a private method below

private void Login()
        {
            int UserId;
            UsersDataModel usersDataModel = new UsersDataModel();
            using (SqlConnection con = DBCS.GetConString())
            {
                string UserName = username.Value.Trim();
                string Password = password.Value.Trim();
                string SchoolCode = lblSchoolCode.Text.Trim();
                string SchoolName = lblSchoolName.Text.Trim();
                string LastLogin = lblLastLoginDate.Text.Trim();
                //UserId = Convert.ToInt32(hfUserID.Value.Trim());

                con.Open();
                SqlCommand cmd = new SqlCommand("Validate_User", con);
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@Username", UserName);
                cmd.Parameters.AddWithValue("@Password", Password);

                SqlDataAdapter da = new SqlDataAdapter(cmd);
                DataTable dt = new DataTable();
                da.Fill(dt);
                RowCount = dt.Rows.Count;
                for (int i = 0; i < RowCount; i++)
                {
                    UserName = dt.Rows[i]["Username"].ToString();
                    Password = dt.Rows[i]["Password"].ToString();
                    SchoolCode = dt.Rows[i]["SchoolCode"].ToString();
                    SchoolName = dt.Rows[i]["SchoolName"].ToString();
                    LastLogin = dt.Rows[i]["LastLoginDate"].ToString();
                    UserId = Convert.ToInt32(dt.Rows[i][""]);

                    if (UserName == username.Value && Password == password.Value)
                    {
                        Session["NickName"] = UserName;
                        Session["SchoolCode"] = SchoolCode;
                        Session["SchoolName"] = SchoolName;
                        Session["LastLoginDate"] = LastLogin;

                        if (UserId == -1)
                        {
                            lblMessage.Text = "Username and/or password is incorrect.";
                        }
                        else if (UserId == -2)
                        {
                            lblMessage.Text = "Account has not been activated.";
                        }
                        else
                        {
                            WebMessageBox.Show("logged in");
                        }
                    }
                    else
                    {
                        lblMessage.Text = "Invalid User Name or Password! Please try again!";
                    }
                }
            }
        }

But this function gives me this error

Column 'Username' does not belong to table .
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: Column 'Username' does not belong to table .

Source Error:


Line 46:                 for (int i = 0; i < RowCount; i++)
Line 47:                 {
Line 48:                     UserName = dt.Rows[i]["Username"].ToString();
Line 49:                     Password = dt.Rows[i]["Password"].ToString();
Line 50:                     SchoolCode = dt.Rows[i]["SchoolCode"].ToString();

Below is my stored procedure:

USE [SchoolMIS]
GO
/****** Object:  StoredProcedure [dbo].[Validate_User]    Script Date: 12/11/2017 7:05:30 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[Validate_User]
      @Username NVARCHAR(20),
      @Password NVARCHAR(20)
AS
BEGIN
      SET NOCOUNT ON;
      DECLARE @UserId INT,
              @LastLoginDate DATETIME

      SELECT @UserId = UserId,
             @LastLoginDate = LastLoginDate
      FROM Users

      WHERE Username = @Username AND [Password] = @Password

      IF @UserId IS NOT NULL
      BEGIN
            IF NOT EXISTS(SELECT UserId FROM UserActivation WHERE UserId = @UserId)
            BEGIN
                  UPDATE Users
                  SET LastLoginDate = GETDATE()
                  WHERE UserId = @UserId
                  SELECT @UserId [UserId] -- User Valid
            END
            ELSE
            BEGIN
                  SELECT -2 -- User not activated.
            END
      END
      ELSE
      BEGIN
            SELECT -1 -- User invalid.
      END
END

Hope someone can give me an assistance.