Error: This webpage has a redirect loop

ERROR:This webpage has a redirect loop
The webpage at http://localhost:12205/kreu.aspx?ReturnUrl=%2findex.aspx has resulted in too many redirects. Clearing your cookies for this site or allowing third-party cookies may fix the problem. If not, it is possibly a server configuration issue and not a problem with your computer.
 
I have created a login system like this but when i run it gives the error
 

protected void btn_login_Click(object sender, EventArgs e)
        {
 

            string strCon = ConfigurationManager.ConnectionStrings["ConnectionStringusers"].ConnectionString;
 
        string strSelect = "SELECT COUNT(*) FROM [Users] WHERE UserName = @Username AND Password = @Password";
 
 
 
        SqlConnection con = new SqlConnection(strCon);
 
        SqlCommand cmd = new SqlCommand();
 
        cmd.Connection = con;
 
        cmd.CommandType = CommandType.Text;
 
        cmd.CommandText = strSelect;
 
        SqlParameter username = new SqlParameter("@Username", SqlDbType.VarChar, 50);
     username.Value = username_id.Value.Trim().ToString();
 
        cmd.Parameters.Add(username);
 

        SqlParameter password = new SqlParameter("@Password", SqlDbType.VarChar, 50);
 
        password.Value = password_id.Value.Trim().ToString();
 
        cmd.Parameters.Add(password);
 
        con.Open();
 
        int result = (Int32)cmd.ExecuteScalar();
 
        con.Close();
 
 
 
        if (result >= 1)
 
        {
 
            Response.Redirect("kreu.aspx");
 
            Session["username"] = username_id.Value;
            
        }
 
        else
 
           Response.Write(@"<script language="'javascript'">alert('Invalid username or password!') </script>");
 
 
 
    } 
     
 
        }
}