Using ab initio methods based on density functional theory, the electronic and magnetic structure of layered hexagonal NbSe$_{2}$ is studied. In the case of single-layer NbSe$_{2}$ it is found that, for all the functionals considered, the magnetic solution is lower in energy than the non-magnetic solution. The magnetic ground-state is ferrimagnetic with a magnetic moment of 1.09 $mu_{B}$ at the Nb atoms and a magnetic moment of 0.05 $mu_{B}$, in the opposite direction, at the Se atoms. Our calculations show that single-layer NbSe$_{2}$ does not display a charge density wave instability unless a graphene layer is considered as a substrate. Then, two kinds of 3$times$3 charge density waves are found, which are observed in our STM experiments. This suggest that the driving force of charge instabilities in NbSe$_{2}$ differ in bulk and in the single-layer limit. Our work sets magnetism into play in this highly-correlated 2D material, which is crucial to understand the formation mechanisms of 2D superconductivity and charge density wave order.
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