No water in my floor heating loop

Okay, to me this one defies logic... I have a zoned floor heating set up and one pump which went to a sun room with no crawl space went bad last spring. I drained the water from the loop so it wouldn't freeze. I have now replaced the pump but I can't get water to flow into the loop. It is fed off a pipe from the water softener. The pipe drops vertically to the boiler and in the middle of that pipe, the sun room zone Ts off, goes through a ball valve, through a back flow preventer, and into the loop. There is water pressure in the boiler, (15 psi) and water in the other pipes that come from the main line from the water softener, however, no water if flowing into the sun room zone loop. I detached the back flow preventer, and no water is flowing to that either. Could the ball valve be stuck in the closed position, even though the handle opens and closes? Could there be too little water pressure to get water into the T section of the pipe? (The pressure from the well is at 50 PSI.) If there was no additional water flowing into the boiler, wouldn't the pressure drop? Any suggestions would be very helpful.