If the autopilot was on, it's a question of how much fuel is on board as related in another answer (unless high ground was in the way).
If the autopilot is off, even if in perfect trim, the aeroplane will usually gradually drop a wing (this process will start within 15-30 seconds) and develop an ever steepening spiral dive and crash at high speed into the ground.
Sometimes something different has happened, perhaps if a dead pilot has his hands on the controls, but in most cases there is insufficient stability for it to remain wings level for long without pilot input.
I'm thinking of an example in the States where an airliner's wing sliced through the cabin of a light aircraft and cut the three occupants into two. That aircraft managed to land itself intact in a school playing field I think it was, with blood covering the fuselage and wings, but no fuselage top and no visible occupants.
Also, in a small plane the trim might be affected by people disappearing (due to a change in CG) but I think the usual result would be to drop a wing as related.
I am not aware of any systems that would turn an autopilot on automatically if it was off. Finally, many light aircraft only have wing leveller autopilots, so in this case, even if it was out of trim on the elevators, it would take longer (perhaps much longer) to crash, it just might fly long enough to run out of fuel, assuming no mountains were in the way.