In my experience the most common answer to this problems is a plugged strainer. This is usually in a tight spot below deck plates or just an ugly job so no one wants to do it, but there is a good chance thats it is plugged, and plugs itself very fast, so no one thinks the 2 minutes they used it will affect anything.
Apart from that you can rattle off a multitude of defective things from the obvious no power to the motor to the obscure leaking mechanical seal and in everything in between like check valves, piping issues, busted gauge, broken belt, broken shaft, etc etc.
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