Superman Through the Ages! Forum

I wonder what happened to Argo City once all its inhabitants died from Anti-Kryptonie poisoning, did it just float through space
It might have been eerie for Supergirl to return there but if she did she would probably lose her powers

The dead Argo City was seen in at least two Pre-Reboot stories.  In "Supergirl #18" (April 1984) , an alien named Kraken from another dimension returned to Argo City, the site of his earlier defeat.  Finding it lifeless, he vowed to track down the last survivor (namely Kara Zor-El) to gain his vengeance.  

And then there's "DC Comics Presents #97" (September 1986), arguably the last "Earth-1 Superman" story though it takes place post-Crisis.  In this wild story, an out-of-control Mr. Mxyzptlk sends Argo City crashing down on Metropolis, burying the city under tons of kryptonite.  (There is a chilling panel of long-deceased Argo City residents falling from the sky amid the green rubble -- one assumes all Kryptonian microbes also perished, leaving the bodies preserved until Argo City drifted closer to a yellow sun and rendered the victims immune to further decay.)

Since the crazy events of "DCCP #97" were immediately retconned by the Bryne reboot, one really wonders if this TRULY belongs in the old continuity.  (But unlike "Whatever Happened to the Man of Tomorrow?", this was NOT labeled as an "Imaginary Story".)  You can read a summary and review of this issue (which also sees the "final end" of Mr. Mxyzptlk, Bizarro World, the Phantom Zone and its prisoners, along with the re-introduction of lethal kryptonite back on Earth at:  

http://www.supermanhomepage.com/comics/pre-crisis-reviews/pre-crisis-mmrs-intro.php?topic=c-review-pc-dccp97