Soumava said:
what happens to the original electromagnetic wave
This is a good question to ask, but I can only give a partial answer without talking about photons.
To get some intuition for what happens to an EM wave as it gets absorbed, you will want to look up how EM waves behave in a medium that can conduct electricity, since here, we have free charges interacting with the field.
Looking at Maxwell's equations, you can argue that for an EM wave in a vacuum, the rate of change of the electric field drives the magnetic field, while the rate of change of the magnetic field in turn drives the electric field.
In a conductor, this simple reciprocal relationship is no longer true, as the magnetic field is driven both by the rate of change of the electric field, and the electric current, which in turn is approximately proportional to the electric field (from Ohm's law).
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This is a hand-wavey way of looking at it, but...
Because the electric field is causing the charges to move, and because moving charges generate magnetic fields, and because accelerating charges generate changing magnetic fields, accelerating charges produce electric fields in the opposite direction of their acceleration, subtracting away from the original electric field, which decreases it.
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Because of the extra dependence on electric current, the EM wave equation you get by taking the curl of both sides of Faraday's law contains an extra term that depends on the rate of change of the current (i.e., the overall acceleration of these moving free charges). This acceleration term acts as a damping term in the wave equation. Indeed, when you solve Maxwell's equations for a plane-wave incident on a conductor, you see the EM wave decrease in intensity exponentially with distance through the medium.
I know this answer is very mathematical, but I hope it helps a little.