If you finished a flight, the fuel remaining at the end of your previous flight would be all you have for your next flight. Even loading a different 757 III model will not reset the fuel, so you need to redo the fuel after completing a flight.

I'm not sure if, by what you are saying, you think that changing/resetting a panel state will reset the fuel. It doesn't. It only resets the panel state.

If none of therse is the case, I have no idea why it tumbled out of the sky. This has never happened to me. I have stalled after takeoff though, but that was because of pilot (me) error!