No. In any reaction, both matter and energy are conserved. It is often stated that in a nuclear reaction, "matter is converted into energy", but this is misleading - since both matter and energy are conserved! A helium-4 atom (for example) does indeed have less mass than than four hydrogen-1 atoms. But the missing mass is found in the energy. For example, at first, in the Sun, this energy will be released as heat - and the additional energy will increase the mass. Or rather, maintain the exact amount of mass there was before. And the energy doesn't just appear out of nowhere - it was there, before, as potential energy.