MoS2 is a promising low-cost catalyst for the hydrogen evolution reaction (HER). However, the nature of the active sites remains controversial. By explicitly considering the electrochemical potential using grand canonical density functional theory (DFT) in combination with the linearized Poisson-Boltzmann equation, we revisit the active sites of 2H-MoS2. We start with the edges and we assess the influence of the presence of water and the competition between the release of H2S and H2. In our calculations, we consider the 50% S-edge and the 50%S Mo-edge, as they have been shown to be the most stable edge structure. Our calculations reveal that the Mo-edge is very likely to be reconstructed in the presence of water. In particular, H2S formation is only weakly endothermic (0.4 eV). Hence, under HER conditions where there is no sulfur reservoir, sulfur is gradually replaced by oxygen. Fortunately, the 0%S Mo-edge covered by OH is active for HER, as shown in our previous study. For the 50% S-edge, the termination is more stable with respect to H2S release (2.5 eV) and OH is stably adsorbed on the Mo-Mo sites. Our grand canonical DFT computations suggest that this edge is active for HER. The release of H2S at this edge is less likely to take place compared to H2 generation but is still possible as it is endothermic by only 0.21 eV after adsorption of OH. Once this OH substitutions happen, it leads to a 0%S S-edge that is predicted to be inactive for HER. For the basal plane, the thermodynamics of proton reduction as a function of electrochemical potential shows that three basal plane defects exhibit thermodynamic overpotentials below 0.2 V3 and the single sulfur vacancy (Vs) is the most abundant defect. Therefore, we present a detailed mechanistic study of HER on this defect and evaluate the Vomer, Tafel and Heyrovsky transition states for the different possible reaction steps by considering the activation energy as a function of the electrochemical potential.