  We carry out a comprehensive analysis of supernova \snem\ covering a period
of 19 years since discovery. SN 2001em is the oldest supernova known to have
undergone a metamorphosis from a stripped envelope to an interacting supernova.
An early spectrum indicates it exploded as a Type Ib supernova. Later, the
ejecta caught up with a dense circumstellar H-shell, ejected a few thousand
years before the explosion, triggering interaction between the supernova ejecta
and the dense shell, producing radio, X-ray and H$\alpha$ emission. We use
archival data with the Very Large Array in radio bands and with Chandra,
XMM-NEWTON and SWIFT-XRT in X-ray bands, along with published H$\alpha$
measurements. We combine these data with our low radio frequency observations
with the Giant Metrewave Radio Telescope at two epochs covering three
frequencies. While the observations missed the phase when the shock entered the
dense shell, the X-rays indicate that the shock came out of the dense shell at
around 1750 days. The data suggest a forward shock origin of the X-ray
emission. Radio data show a spectral inversion at late epochs ($> 5000$\,d) at
around 3 GHz, which mimics the properties of the central absorbed component
seen in SN 1986J. A possible explanation for this component is that the
progenitor of \snem\ was a massive binary system which had undergone a period
of common envelope evolution. The hydrogen envelope from the \snem\ progenitor
may have been lost as a result of binary interaction.
