usually when a city is destroyed by war or natural causes like flood or earthquakes, another version is rebuilt on top of the ruins, often using salvage from the previous construction.
I find it very curious that nothing was rebuilt on the site and yet the foundations of stone buildings are being uncovered. Looking at the model in the museum I would think someone would come up with a theory about the abandonment of this sturdy ancient city.
[snip] Viminacium was devastated by Huns in the 5th century, but was later rebuilt by Justinian. It was completely destroyed with the arrival of Slavs in the 6th century. [/snip]
https://en.wikipedia.org/wiki/Viminacium
https://en.wikipedia.org/wiki/Moesia#History