Often the city inlet check valve simply gets a bit of dirt stuck in it, as John described. If the repair doesn't work, just put a threaded plug on it for now. It's standard garden hose thread. Rv stores will have fancy ones (see link below), but you can get a 3/4" hose thread plug at any hardware store or dept.
http://www.campingworld.com/shopping/item/water-inlet-plug/27840