I would assume you have a check valve after the pump, or water would flow back from the returns. If water does not flow from the suction side, there must be a blockage(s). Is there a main drain and skimmer? Separate lines or joined underground, most likely at the skimmer? The latter is my assumption, but in any event I would have a local pool guy come vac out the suction lines to try and clear any blockage. Mind you, by the time I am writing this, your pump may have pulled through a pile of leaves etc already