Military action
"Vying for power, the peoples of Hijr waged war against tone another. Trying desperately to claim the former Uldoth lands and technologies for themselves, the peoples descended into barbarism." - 'Of Iron and Dwarves', by Nassim Latal
With the sudden and unexpected disappearance of the Uldoth, the remaining races of the world tried to maintain the levels of power and technology they had previously known... However without the guidance of their progenitors, the younger races descended into barbarism and infighting. The descendants of the Uldoth did battle with one another, Humans fighting against Goblin-kind, and Dwarves declaring war on the Orcs... All the while the descendants of the dragons waged war on them all. And the descendants of the Ellef colonies on the Prime Material Plane did their best to stay out of the conflicts, with little success. This war waged for hundreds of year, and most of the technology and constructions of their forebears were lost, leaving the world in a primitive and tribal state. It was only a final treaty between the most powerful races, and the founding of an Elf and Human nation, that finally ended the majority of the fighting, though some races never truly gave up the feud.