How to submit the correct request?

Question

API

string url = "api.openweathermap.org/data/2.5/weather?q=" + Cities + "&appid=" + api + "&lang=ru&units=metric";
            var oi = new HttpRequest();

            string op = oi.Post(url).ToString();
            var objects = JObject.Parse(op);

Yesterday this code worked, today it throws the error "xNet.HttpException: "Could not send request to HTTP server 'api.openweathermap.org'.""