ASP.Net Error: An object reference is required for the nonstatic field, method, or property

I am writing this code to get url from database but is showing error how to fix it?

    static string SendSMS(string msg, string receipietnt, string mask)
    {
        con = new SqlDbConnect();
        con.SqlQuery("select Link,AKey,UserName,Password from tblSMSSetup");
        con.RdrEx();
        if (con.Rdr.Read())
        {
            
            string link = con.Rdr["Link"].ToString();
            string key = con.Rdr["AKey"].ToString();
            string userName = con.Rdr["UserName"].ToString();
            string password = con.Rdr["Password"].ToString();
            string download = "";
            try
            {
                string apipart1 = string.Format("http://api.smssky.pk/api-send-branded-sms.aspx?username={0}&pass={1}", userName, password);
                string apipart2 = "&text=";
                string apipart3 = "&masking=";
                string apipart4 = "&destinationnum=";
                System.Net.WebClient myWebClient;
                myWebClient = new System.Net.WebClient();
                download = myWebClient.DownloadString(apipart1 + apipart2 + msg + apipart3 + mask + apipart4 + receipietnt + "&language=English");
                int st = download.IndexOf("<span id=\"lblmessage\">");
                int end = download.LastIndexOf("</span>");
                download = download.Substring(st, end - st);
                download = download.Replace("<span id=\"lblmessage\">", "");
                download = download.Trim();
                download = download.TrimEnd();
            }
            catch (Exception ex)
            {
                download = ex.Message;
            }
        return download;
        }
        con.Rdr.Close();
        con.conClose();
    }