Crash: China Eastern B738 near Guangzhou on Mar 21st 2022 (opens in new tab)

It depends on exactly what has happened to cause a lack of control. Essentially all fixed-wing aircraft other than high-performance military fighter jets are built to be aerodynamically stable: if you do nothing, they go in a straight line and if they are in an established turn, they tend to continue in that turn. There are plenty of examples of "famous" not-as-bad-as-they-could-have-been crashes where control was effected unconventionally in aircraft made otherwise uncontrollable by mechanical failure -- for example, using throttle control to land aircraft experiencing their phugoid nose up / nose down inherent dynamic stability mode [1, 2].

Full blown nose down is rare unless controlled flight into terrain. The obvious armchair speculation I would make here is that a total separation of the tailplane / stabilator or empennage of the aircraft could cause something approximating that -- the tailplane provides negative lift and without it the aircraft would pitch up until it stalled, and then a series of exponentially growing oscillations would likewise result.

It's far better to wait for the NTSB accident report and deliver sympathy and aid to those affected, however -- we don't have the facts and I am sure that they will be determined in the fullness of time.

[1] https://en.wikipedia.org/wiki/United_Airlines_Flight_232 [2] https://en.wikipedia.org/wiki/2003_Baghdad_DHL_attempted_sho...