you should always keep the pump submerged, once you begin using it (including when it's off/overwintered), so any seals don't dry out. Too, once dry, rust, debris etc can seize parts of the motor and cause the impeller to break the next time you power it up. A friend of mine did this once--motor was fine (humming) but no water was pushed. If you turn it on and it hums but no water, suspect impeller break. If you power it on and there's nothing, I'd suspect electrical failure and you'll need a new one. If your unit has a thermo cut off, it should reset and save your pump. See if this is a trait of your pump in particular by checking the manual/web site. Generally, if the impeller is blocked, you'll burn the motor up fast.