I have changed an oven element but it doesn't heat. WHY?

The older/cheaper your oven is, the simpler the problems tend to be. On those kind of ovens, the next thing to check would be the temperature control itself, which is controlled by the knob you turn to turn it on or adjust the temperature.

There's a temperature sensor that could be part of the problem as well.

Newer/more advanced models will often have a control board that could also be part of the problem.

However, there's no way for us to be able to give you any more definitive help without knowing the manufacturer and model number of your oven; there are just too many different kinds of stoves out there for us to even be able to give you a good guess without that information.