There are many other consequences of relativity besides length contraction and time dilation. Perhaps the most famous and most important of these concerns energy.
Energy comes in many forms.
This energy depends on the speed of the object, and on its mass. Heat is also a form of energy, which can ultimately be traced to the kinetic energy of the individual atoms and molecules that make up a substance. And there are many other forms of energy. In other words, energy is neither created nor lost, even though it may be converted from one form to another. I've pointed out that a moving object has energy due to its motion.