We present results of first-principle calculation of the electronic structure and phase stability of the parent compound of Fe-based superconductors, FeSe, in a magnetically ordered state. In particular, we investigate ferromagnetic (FM) and two different types of antiferromagnetic (AFM) configurations (with magnetic structure vectors (π, 0) and (π,π)). Our results for the total energy exhibit a two-minima shape for the FM and a standard parabolic-like behavior for the AFM configurations. We find a remarkable reconstruction of the electronic structure in the vicinity of the M point of the Brillouin zone which is accompanied with a rapid increase of magnetic moment upon expansion of the lattice volume. On that basis we propose that both the anomalous behavior of FeSe upon expansion of the lattice reported for the paramagnetic state (Phys. Rev. Lett. 115, 106402 (2015)) and that obtained in the present work have a common origin.