Todaymy horse has one eye closed when I opened it his eye lid was red and the center is cloudy12 yr help!!!!!?

I just got back from vacation and went to see my horse today. I noticed he had his left eye closed. When we got in the barn I opened it and his eye lid is VERY red and the center of his eye is cloudy. He is a 12 yr old thoroughbred with no history of health issues. HELP ME!!!!