the crankshaft sensor tells the computer that the engine is running. It's the RPM counter. If you don't get any signal from the crankshaft sensor the computer will see that the engine is not turning over so you won't get any spark and you won't get any fuel injection pulse.
so if you are trying to start a vehicle, it will cause a "crank, no start" condition. If the engine is running and loses the crank signal it will cause the engine to stall. If your vehicle sees a cam signal but doesn't see a crank signal, it will set a code. (the check engine light will come on) If you have no crank and no cam signal, the check engine light may not come on because the computer doesn't see the engine cranking at all.