Answer
The conservation of energy law would be violated.
Work Step by Step
A proton has a rest energy of $938.3~MeV$
A neutron has a rest energy of $939.6~MeV$
A neutrino has a rest energy of approximately $1\times 10^{-7}~MeV$
Since the total rest energy of a neutron and a neutrino is greater than the rest energy of a proton, and we assume that the proton is at rest, the law of conservation of energy would be violated.
Therefore, if a proton decayed into a neutron and a neutrino, the conservation of energy law would be violated.
