What Causes Undescended Testis In Children?

Hi... Recently when I went to take medicine for my son, doctor told he is having undescended testis. He is of 7 years old now. But all of sudden it was the shocking news to me. Because we have been showing him to the doctor many times but we never diagnized by doctor or anyone. Is it possible to happen this all of sudden. Or is there any chances that the doctor would have missed to notice this during early check ups. And is this will be abnormal during climatic abnormalities. Is there any chance that it will be ok with the normal climatic temperature. Whether surgery is necessary for this or is there any other treatment available for this?