The answer is that the mass was destroyed and releases as kinetic energy. D m is the mass deficiency, and c is the speed of light. This is a lot of energy. An important parameter for nuclear stability is the nuclear binding energy divided by the number of nucleons. For example, nuclear fusion involves forcing two mutually repulsive nuclei to come within a very short distance of one another.
Fission is somewhat easier to induce.
Often the introduction of a neutron to a nuclide is enough to destabilize it, causing fission. Note also that while one neutron disappears, two more appear. If conditions are right, the two daughter neutrons can also be absorbed, producing even more neutrons. It is possible that there can be a literal explosion of neutrons and, as a consequence, energy.