There are planes that have plummeted.. is that because of some other gross inconsistency? Pilot pushes nose down or something... Surely there's a point of no return from a plane just being a large glider to it falling nose first to the earth.
An aircraft can 'plummet' for a couple of reasons. The most common example of 'plummeting' is when an aircraft flies into vertical windshear and this is where it flies into a volume of air that is rapidly moving downwards, carrying the aircraft down with it. Less commonly, an aircraft can 'plummet' if it flies into horizontal windshear, where the air is moving in the same direction as the aircraft. In this case, the airspeed of the aircraft can drop to the point where it is insufficient to produce lift.
Funnily enough, if the nose of an aircraft is pointing to Earth, you're soon going to have enough airspeed to regain lift and pull out of the dive, structural limitations permitting. It's actually when the nose isn't pointing towards the Earth and you're falling that's the problem.
As we're talking about airliners here - they were mentioned in the original question - what would happen if the engines stopped is that the aircraft would start to slow down due to drag and friction. As the aircraft slows down, the wings will produce less lift, so if the pitch of the aircraft is unchanged the aircraft will start to sink. However, airliners need a lot of power to keep them in the air, so they lose speed and lift rapidly, and at a certain point the aircraft will be going too slow, even though it's descending, for the wings to produce enough lift to keep the aircraft flying and it will 'stall'. At this point, the aircraft would then 'plummet' or simply fall out of the sky.
If the engines failed, the pilot could try to compensate for the loss of lift due to the reduction of airspeed by increasing the pitch i.e. by raising the nose and increasing the angle of airflow over the wings, which increases the amount of lift they produce, but the downside to this is that this also increases the drag, so the aircraft slows down even more rapidly and stalls even sooner.
What the pilot has to do then, is achieve a rate of descent, at the optimum pitch, such that (s)he gains/maintains sufficient airspeed over drag and friction to preserve lift.