Okay, I had to dig it up from my collection, but Clark's resurrection is explained by Dr. Occult in Action Comics 692.
And so I don't reprint the text of the entire book here, Occult goes on to explain how Jonathon Kent convinces his son to fight his way back to the land of the living, and how the Eradicator places Clark's body in the "Kryptonian Regeneration Matrix" and eventually sacrificed his life to save Clark from the blast of the deadly Kryptonite X.
p19. Dr. Occult: Make no mistake, his spirit had fled his body. The energies stored within the body preserved it temporarily--
--Still, sealed off in that tomb, it would eventually have perished. And then, there could have been no return.
He needed the light of the sun, not the dark of the grave.
With his body in that null state, his spirit was trapped between the living and the dead... and became prey for all manner of demons.
p20. Lois: Are you saying... if Superman is ever about to... to die again... all we have to do is make sure he gets plenty of sun--?
p21. Dr. Occult: No, it's not that simple. Superman was very lucky... this time.
Never forget, his spirit had left the body. A whole chain of events -- physical and metaphysical -- combined with things both of the Earth and of Krypton to restore him to life...
...And some of those elements no longer exist. Your will to live is strong Superman, but your resurrection is not a repeatable phenomenon. You are mortal, and someday you will die... as must we all.
So, biologically speaking, he was only mostly dead, but well on his way to being all dead. Spiritually, he was dead, dead, deadinsky, but not passed on to his final reward. And apparently, a lot of sun won't really help.
(I hope I haven't broken any rules by the amount of text I quoted. Not sure how that works exactly.)