The LHCb experiment has the unique property to study heavy-ion interactions in the forward region (2 < η < 5), in a kinematic region complementary to the general purpose detectors. The detector has excellent capabilities for reconstructing quarkonia down to zero pT. Notably, it can separate the prompt and displaced components. In pPb collisions, both forward and backward rapidities are covered thanks to the possibility of beam reversal. Results include measurements of the nuclear modification factors and forward-backward ratios for charmonium states. These quantities are sensitive probes to study cold nuclear matter effects on quarkonium production. In 2015, LHCb also participated successfully for the first time in the Pb-Pb data-taking. The status of the forward prompt J/ψ nuclear modification factor measurement for up to semi-central lead-lead collisions will be shown.