Incorrect syntax near '.'.

Hi,
I am working with SQL Server 2008 R2 show me this erorr.if som one know about this please tell me?
Incorrect syntax near ','.
using System;
using System.Collections.Generic;
using System.Configuration;

using System.Data;

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;

using System.Data.SqlClient;
using System.Data.Sql;
using System.ComponentModel;
using System.Drawing;
using System.Text;

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
txtCaptcha.Text = "";
FillCapctha();
}
}
void FillCapctha()
{
try
{
Random random = new Random();
string combination = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
StringBuilder captcha = new StringBuilder();
for (int i = 0; i < 6; i++)
captcha.Append(combination[random.Next(combination.Length)]);
Session["captcha"] = captcha.ToString();
imgCaptcha.ImageUrl = "GenerateCaptcha.aspx?" + DateTime.Now.Ticks.ToString();
}
catch
{

throw;
}

}
protected void btnRefresh_Click(object sender, EventArgs e)
{
refresh();
txtCaptcha.Text = "";
FillCapctha();


lbl.Text = "";


}
public void refresh()
{

txtName.Text = "";

txtEmailId.Text = "";

txtMobileNo.Text = "";

drpdnState.Text = "";

drpdnCity.Text = "";


txtCaptcha.Text = "";


}
protected void btnSendRep_Click(object sender, EventArgs e)
{
if (Session["captcha"].ToString() == txtCaptcha.Text)
{
SqlConnection con=new SqlConnection();
con.ConnectionString ="Data Source=WIN-Q3FHUVKHP7O\\SQLEXPRESS;Initial Catalog=Demat;Integrated Security=True";
con.Open();
SqlCommand cmd = new SqlCommand("insert into DematUsers(Name,Mobile,Email,State,City,Captcha,LoginTime) values('" + txtName.Text + "','" + txtMobileNo.Text + "','" + txtEmailId.Text + "','" + drpdnState.Text + "','" + drpdnCity.Text + "',,'" + txtCaptcha.Text + "''" + dt12.Text + "')", con);
cmd.CommandType = CommandType.Text;
try

{



cmd.ExecuteNonQuery();

lbl.Text = "Data Inserted";

con.Close();




refresh();



}
catch (Exception ex)

{

lbl.Text = ex.Message;

}

}
else{

Response.Write("<script>alert('Captcha Code Invalid!! ')</script>");
FillCapctha();
}

}
}