Car was run out of oil due to plug not installed properly. Now harmonic balancer has come off. Can this be due to warping of the crank shaft?
My guess is running out of oil caused the crankshaft to spin in a dry environment and that generated friction and a whole lot of heat. The heat melted the built in rubber isolator within the harmonic balancer.
So no the crankshaft isn't warped. It will just have heavily scratched and scored bearing journal surfaces.
But I'm getting the impression you are not telling us everything. If you ran the engine for more than 45 seconds without oil I would think the entire engine needs a total rebuild. I wouldn't think that replacing just a harmonic balancer would get you up and running again. What's the goal here?
Technically it shouldn't be the reason. But talking about your crank - if you're running out of oil you have less oil, so it doesn't have a time to cool down. That leads to loss of a pressure. If you catch an air bubble, it reduces the pressure for a moment, but returns. If you're a slow driver, I'd suggest to check a pressure. If your Oil warning light comes up at any time, it means your bearings are damaged. If you love to rev it up, I'd suggest to check your mains.. Lift the car up, take the sump off, undo the main bearings and see the condition. Use a torque wrench for mains when assemble it back..