Superman's body had died. There was no question of that fact.

Fortunately, however, a series of events and instances combined to make his return to life possible.

There was the fact that Superman's body was a living solar battery, and had been absorbing solar energy his entire life. Even after his body's death, there was still a massive amount of energy stored within his cells.

Then there was the theft of his body from the tomb by the Eradicator. The Eradicator had pulled its energies back together following its latest defeat at the hands of Superman, and had come to claim the Man of Steel's body and return it to the Fortress of Solitude. There, the Eradicator placed the body in a Kryptonian regeneration matrix. The energies of the matrix, combined with the energy still within Superman's body, was able to restart the body's biologic functions.

And there was the intervention of Jonathan Kent, who, having an out-of-body experience following a heart attack, was able to persuade his son's soul to return to his body.

So Superman returned to life. Unfortunately, his "re-birth" from the regeneration matrix was so violent that the matrix was destroyed. Alive but nowhere near his previous level of power, Superman required the assistance of Supergirl, Steel, and Superboy to battle Mongul and the Cyborg at Engine City. One final piece was needed to return the Man of Steel to his former strength.

That piece was supplied by the Eradicator, who took the brunt of a kryptonite explosion and channeled the radiation through his body into Superman's. This had the immediate effect of returning Superman to full power. However no one could have predicted the surprising (and near-fatal) consequence of the Eradicator's actions.