Fix First-Data Payment Gateway Issue

Hello I have a website here [login to view URL] Someone installed First Data Payment Gateway and we tested it and everything was working OK but now it won't work - this might be either a First-Data installation error of very likely a PHP error (Not connecting the files properly) When you try to make a payment the form does not respond, earlier it was bringing the warning message saying "headers have already been sent" I want to work ONLY with people who have dealt with FIRST DATA payment gateway installation. My budget is fixed but if it takes more work I will pay little extra because I need to get this done before Monday morning 1/28 Thanks