Custom SharePoint WebServices project call from stored procedure HTTP 500 error

Hi,

I created new solution to SharePoint 2013, where I generate new webservice - update my document library item from erp system

If I call this webservice manually, then work fine, but If called from programatically, than  get HTTP 500 error

I'm new in webservices and I don't no where is the error

 public class SPWebService : WebService
    {
[WebMethod]
        public string setAttachment(int id, string path, int bid, int btid)
        {

            try
            {
                SPSecurity.RunWithElevatedPrivileges(delegate()
                {
                    using (SPSite site = new SPSite("http://mysp"))
                    {
                        using (SPWeb web = site.OpenWeb())
                        {
...
                        }
}

Please help!

Thanks,

Mykie