First stick some weed whacker string up the hole to make sure it's not clogged. Critters will sometimes build homes in there. If that's not it, then disassemble and check the pump, especially if it hasn't had a new one in recent memory. The impellers are rubber, the rubber will get stiff and stop working, and in a worst case the rubber can come apart. They need to be replaced periodically, even if the motor hasn't been used much.