  We report the first detection of hard (>10 keV) X-ray emission simultaneous
with gamma rays in a nova eruption. Observations of the nova V5855 Sgr carried
out with the NuSTAR satellite on Day 12 of the eruption revealed faint, highly
absorbed thermal X-rays. The extreme equivalent hydrogen column density towards
the X-ray emitting region (~3 x 10$^{24}$ cm$^{-2}$) indicates that the shock
producing the X-rays was deeply embedded within the nova ejecta. The slope of
the X-ray spectrum favors a thermal origin for the bulk of the emission, and
the constraints of the temperature in the shocked region suggest a shock
velocity compatible with the ejecta velocities inferred from optical
spectroscopy. While we do not claim the detection of non-thermal X-rays, the
data do not allow us to rule out an additional, fainter component dominating at
energy above 20 keV, for which we obtained upper limits. The inferred
luminosity of the thermal X-rays is too low to be consistent with the gamma-ray
luminosities if both are powered by the same shock under standard assumptions
regarding the efficiency of non-thermal particle acceleration and the
temperature distribution of the shocked gas.
