  Searches are performed for both prompt-like and long-lived dark photons,
$A^{\prime}$, produced in proton-proton collisions at a center-of-mass energy
of 13 TeV, using $A^{\prime}\to\mu^+\mu^-$ decays and a data sample
corresponding to an integrated luminosity of 1.6 fb$^{-1}$ collected with the
LHCb detector. The prompt-like $A^{\prime}$ search covers the mass range from
near the dimuon threshold up to 70 GeV, while the long-lived $A^{\prime}$
search is restricted to the low-mass region $214<m(A^{\prime})<350$ MeV. No
evidence for a signal is found, and 90% confidence level exclusion limits are
placed on the $\gamma$-$A^{\prime}$ kinetic-mixing strength. The constraints
placed on prompt-like dark photons are the most stringent to date for the mass
range $10.6 < m(A^{\prime}) < 70$ GeV, and are comparable to the best existing
limits for $m(A^{\prime}) < 0.5$ GeV. The search for long-lived dark photons is
the first to achieve sensitivity using a displaced-vertex signature.
