Airless Paint sprayers have two check balls that push the paint through the system. If one or both are either blocked open by anything or frozen on their seats by dried paint or hard water, the machine will do what you are describing. First with the machine running in water, try hitting the pump unit smartly with a hammer. The object is to knock the ball(s) lose without damaging the pump. If it frees up and starts pumping, run a cleaner through the unit to get everything out. If it does not start pumping, you are going to have to go inside to find out if the check balls are blocked or if the packings are damaged to the point they won't move paint anymore.