Most times when a part of the speed control fails the treadmill simply stops working or, if it is too erratic, most will shut down to prevent further damage or potential safety concerns. Unfortunately, any component can fail and if it is one that directly controls the speed and/or safety systems then some strange things can happen. So it is pretty rare for the speed to suddenly increase by such a large amount but not impossible and, obviously, it could make the treadmill unsafe to use so it does need to be addressed.
If the walk belt or board are too dry or some other issue is causing excess drag it can usually be felt as a minor hesitation or small speed changes as the system tries to compensate for the excess load changes. But, if the controller were overloaded for an extended time, due to excess walk belt drag or some other problem, then it could have been damaged and now makes much more radical speed changes.
Another possibility is an issue in the motor itself that causes the controller to over-react or possibly a problem with the console that causes the input signal to the controller to change radically when it should not.
You could unplug the treadmill then check for any signs of shorted, broken, or otherwise damaged wiring between those main components. You can also try recalibrating the speed controller if your manual has instructions on how to do so. That might reset the board but be aware that the problem could still reoccur if some other factor is affecting the controller so even if it improves the treadmill still needs a thorough and complete inspection.
Testing the components themselves requires some experience and detailed specifications specific to each model. If tested incorrectly the components could be damaged further or even shock you, so if basic belt maintenance has no effect and no obvious discolored or broken items are noted then you may want to consult a professional to be sure the treadmill is return to a safe operating condition. The links below have more information that might be helpful.