electrical range - oven temperature not set
The oven control problem could be either the temperature sensor or the thermostat. 

In the case of the temp sensor the heat control system looking for an input from the sensor to indicate that the oven is heating up will never get that indication from a broken sensor. It will thus keep heating away with continued hope to get a reaction out of the temp sensor.

In the case of the thermostat the control mechanism may be broken in an number of ways depending upon the type of control that it is. Some thermostats use direct electrical contacts to gate power to the heating element and these contacts can sometimes arc and weld shut causing power to be continuously applied to the heating element. Another thing is that some thermostats use a flexible metal strip as part of their mechanism that responds to temperature or pressure. These flex with usage and can eventually fatigue to the point of no longer moving.

In both cases the faulty part would need to be replaced. You may find it advisable to replace both, particularly if the oven unit is quite old.