Do you only have one pipe that drains into your sump or two? As others have said, we need a bit more info on your setup. If you only have one drain pipe, how big is it? Does it have a valve on it? How big is your return pipe and pump?
If you only have one drain pipe it is possible that it got clogged with a snail or algae or fish. Without an emergency drain pipe you would be in trouble in such and event. Have a look and poke something down your drain pipe to check for obstructions.
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