If you shut it down right when you saw the pressure drop I'd think it'd be ok still....as it wasn't run with 0 oil circulating. Added to that, you had it running, was it running normally? If so I'd suspect the engine is ok but something else is amiss and preventing it from starting. An inspection/compression check could verify that the engine is still ok. You said it won't crank? as in it tries to but can't? You could try turning the engine manually with a wratchet on the crankshaft bolt.........I'd definatley investigate further.......

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