  We present the results from our seven-year long broad-band X-ray observing
campaign of SN\,2014C with \emph{Chandra} and \emph{NuSTAR}. These coordinated
observations represent the first look at the evolution of a young extragalactic
SN in the 0.3-80 keV energy range in the years after core collapse. We find
that the spectroscopic metamorphosis of SN\,2014C from an ordinary type Ib SN
into an interacting SN with copious hydrogen emission is accompanied by
luminous X-rays reaching $L_x\approx 5.6\times10^{40}\, \rm{erg\,s^{-1}}$
(0.3--100 keV) at $\sim 1000$ days post explosion and declining as $L_x\propto
t^{-1}$ afterwards. The broad-band X-ray spectrum is of thermal origin and
shows clear evidence for cooling after peak, with $T(t)\approx 20 \,{\rm
keV}(t/t_{\rm pk})^{-0.5}$. Soft X-rays of sub-keV energy suffer from large
photoelectric absorption originating from the local SN environment with
$NH_{\rm int}(t)\approx3\times 10^{22}(t/400
\,\rm{days})^{-1.4}\,\rm{cm^{-2}}$. We interpret these findings as the result
of the interaction of the SN shock with a dense ($n\approx
10^{5}-10^{6}\,\rm{cm^{-3}}$), H-rich disk-like circumstellar medium (CSM) with
inner radius $\sim2\times 10^{16}$ cm and extending to $\sim 10^{17}$ cm. Based
on the declining $NH_{\rm int}(t)$ and X-ray luminosity evolution, we infer a
CSM mass of $\sim(1.2\,f$--2.0$\sqrt{f}) \rm{M_{\odot}}$, where $f$ is the
volume filling factor. Finally, we place SN\,2014C in the context of 119
core-collapse SNe with evidence for strong shock interaction with a thick
circumstellar medium and we highlight the challenges that the current mass-loss
theories (including wave-driven mass loss, binary interaction and line-driven
winds) face when interpreting the wide dynamic ranges of CSM parameters
inferred from observations.
