I don't know your particular model, but in most cases the pump will have some type of filter between the tank and the pump. Sometimes it's a separate piece, other times it's actually part of the pump. There is usually a small, SS screen inside that can get gunked up. Clean it out and retry. The other thing that can get gunked up would be the actual spray nozzle - do the same thing - take it apart and clean it. Even try the pump without the nozzle on, if you want. Past that, you're looking for an air leak (fittings not connected fully) or the pump is shot. It could be that the last time the system was winterized, they disconnected the line from the tank to do the job.