Fans may not know Superman died in order to do what everyone desires most: cancel plans. In 1992, DC Comics famously killed Superman in the riveting Superman #75, which portrayed his death inflicted by Doomsday through stunning single-panel pages. The issue, since then becoming quite iconic, was actually an idea to postpone Lois Lane and Clark Kent's wedding in reaction to a Superman television series that was in the works that largely focused on the couple. In order to have the wedding in the comics and the wedding in the television series match up for peak marketing, the team behind Superman had to find a big event to replace the wedding within the comics. Jerry Ordway jokingly suggested they "just kill 'im!" And DC wound up doing just that.
Though it began as a joke, The Death of Superman's plot was so pivotal that it was recently reimagined with Lois Lane in the starring role, and has been made into films, both as an animated movie, and as an inspiration to the plots of the recent Batman v Superman: Dawn of Justice and Justice League live-action movies. But for those who have always wondered how did Superman actually die? We have you covered.
The death was basically an act of sacrifice to stop Doomsday from destroying Metropolis and Earth in general. Doomsday, who had his first appearance in 1992, is a gargantuan monster who was genetically-engineered from the depths of a prehistoric Krypton; he was given only feelings of hate and a desire for destruction. Because of this, he destroyed world after world until he landed on Earth. Superman was forced to use everything he had to defeat the villain after Doomsday took out the whole Justice League in mere minutes and ran right for Metropolis' heavily populated streets.
Stopping Doomsday fell on Superman, and it was no easy task considering how hellbent Doomsday really was on killing the Kryptonian. This insatiable desire for death was reasoned in the Hunter/Prey miniseries to be due to "the agony of continually dying during his creation process, Doomsday developed in his genes the ability to sense anyone Kryptonian, as well as an overriding instinct to treat any such being as an automatic threat.” Ultimately, thanks to their matched strength, Doomsday and Superman beat each other to death in front of the Daily Planet office, sufficiently stopping Doomsday's reign of terror on Earth but also taking away one of its greatest heroes. This fight was watched with heartbroken eyes, as panels of a tearful Lois witnessing the battle and Clark's shocked parents watching the coverage on TV made the issue that much more emotional for readers.
Then, it was time for Superman's return/revival! This part was much more peculiar. In reaction to the death of Superman, several phony Supermen debuted in Metropolis to be the new head honcho as a part of the "Reign of the Supermen!" storyline (which was recently made into an animated movie). Alas, half of them were evil but the other half were luckily genuine.
The Eradicator was one of these Superman wannabes, but he's also a former Kryptonian weapon that is trying to protect what's left of the fallen Krypton culture. He stole Superman's body from the crypt it had been placed in and moved it to the Fortress of Solitude, where there was a Kryptonian device called the "Regeneration Matrix." This device allowed the Eradicator to siphon off of Superman's recovering energies in order to power himself while Superman was actively healing, but the matrix quickly broke open and a revived (though weak) Superman emerged. It was emphasized by the Eradicator, however, that the matrix would almost certainly not work a second time...a warning for Clark Kent to not die again.
To make a long story short, Superman and Doomsday fought to the death, before the Kryptonian Regeneration Matrix zapped Superman back to life after the Eradicator stole his body and brought it to the Fortress of Solitude. And all of it was thanks to a wedding needing to be postponed for the sake of marketing!