In the Old Empire, magic was so advanced and accessible that it was equivalent to modern technology. Everyone had access to all kinds of magic items, things kind of like smart phones and the like. Unlike our technology, though, everything was powered by a magical broadcast field. The field made it quick and easy to create magical items that didn't have to be imbued specifically with their own magical energy, and instead just channeled an existing power being broadcast from the Source along a series of existing magical ley lines.
250 years before the campaign begins, every single magical item powered in this way, including the ones that kept the amazing floating cities aloft and the ones that held back the oceans and allowed people to live in areas that should be logically underwater (or kept the domes of the far frozen north warm) all simultaneously and catastrophically exploded.
Almost every person of wealth or power, all of the wizards and warlocks and most of the clerics and all of the "middle class" and even blue collar general workers, almost all of them killed in the same moment in a cataclysm of fire and death.
The survivors were the lowest of the low, simple workers who couldn't afford technology, criminals, weird cultists living without magic in the hills, and a few mad wizards who refused the dominant paradigm of magic. The Empire of the gentle races (elves, dwarves, humans, halflings, gnomes, etc) had driven the dark races deeper into the underdark, so they were on the ropes as it was. Monsters were beaten back to the deep wilderness, many nearly extinct or kept under careful control and bred for their strange properties. Suddenly many of these found themselves free.
This wasn't without other consequences either. During the cataclysm a multicolored Prismatic Storm, a region of shifting colored lights, each representing a deadly area of magical energy, ignited along all of the magical ley lines, stretching high into the sky and deep under the ground and isolating different regions of the world from each other. The characters begin in a far corner of the Empire, an area previously concerned with mining and timber harvesting for luxury goods. As the campaign begins the characters are hired by the concerned Mayor of a mining town and sent to investigate a missing road repair team.