Quasireal photons exchanged in relativistic heavy ion interactions are powerful probes of the gluonic structure of nuclei. The coherent J/ψ photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon (W_{γN}^{Pb}) over a wide range of 40<W_{γN}^{Pb}<400 GeV. Results are obtained using data at the nucleon-nucleon center-of-mass energy of 5.02 TeV collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 1.52 nb^{-1}. The cross section is observed to rise rapidly at low W_{γN}^{Pb}, and plateau above W_{γN}^{Pb}≈40 GeV, up to 400 GeV, entering a new regime of small Bjorken-x (≈6×10^{-5}) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.