What will happen if electrons in an atom stop revolving?

Electrons don't really revolve, it's more a model to imagine an atom. An electron is a negative electrical charge which interacts with the core. Its mass is negligible. If its not around the core, the material which consists of this atoms would collapse, becoming as dense as neutron stars. read more

Practically, I don't think that this can ever happen. But, theoretically speaking, the electron will just fall into the nucleus and some amount of energy will be liberated, due to the presence of nucleons. read more

That's what should happen if electrons fell into a nucleus. A proton's mass is 1.6726 x 10-27 kg, and an electron's mass is 0.00091 x 10-27 kg, but a neutron's mass is 1.6749 x 10-27 kg. So the mass of an electron and proton combined is still nowhere near enough for a neutron. read more