Pangaea existed here on Earth, and over time ours split into the continents that we know today. Eventually, they may return to a different Pangaea like formation, though possibly different than the one before it.
So as long as your world has those same tectonic plates, such a split is possible. Now for the (somewhat apocalyptic) solution, given that there is a fantasy tag in play here ...
Magic went horribly awry
What that magic was exactly, and how that relates to the magic now is not necessarily for the readers to know. What is known by the layman is that there was a calamity and the land heaved and the earth was torn asunder. Lands fled from each other and gaping waters flooded in to keep the lands separated. Who did it is up to you and the needs of your story.
The power of what happened, either by design, through failsafe, or through pure random chance, surged through the tectonic plates of the world. The process accelerated the natural process of your world's Pangaea splitting, causing the appearance of the land heaving. Effectively, your planet's tectonic activity has gone through millions of years in only a few years
Of course, such an abrupt shift in the world is bound to basically be an apocalypse waiting to happen. That is where the runaway magic comes in. The very magic of the world itself was consumed in preventing the end of all life as we know it by severely dampening the volcanic activities that creates the new lands, and preventing the seas from dropping in the chasms opened in the plates in the oceans, and all the immediate problems. There were still natural disasters, but compared to that they should have been -- the world got off lightly.
But not everything can be stopped. Some land masses will move into more inhospitable areas and people there may die out through starvation or predation. Entire villages will disappear into the earth where the fault lines gaped open and left naught but a void and doom. Floods and other disasters caused buy the shifting lands will rewrite the world, causing upheaval.
Depending on where the landmasses travel and how people were spread out over your supercontinent, will help determine those that would perish sooner over later.
The epicentre of the calamity, naturally, has been destroyed beyond recognition
However such an calamitous endeavour has left the magic of the world weak and recovering very slowly, even as the aftershocks of a calamity long ago still rumble across the lands. Magic has been left permanently scarred and weaker, in the low fantasy state that you are desiring.
Without modern technology to fail, the people of the world can carry on without too much interruption to their skill sets. The main problem is that crops that might have thrived before could fail while others that failed could now thrive. But the practices of farming are still similar, and we humans are tenacious if nothing else.
Certain resources may be an issue in these spread out lands, which could be an interesting world building point.
The lack of technology at the time of the calamity means that there won't be a backslide in that regard. People will adapt to the new landscape and climate, as will the flora and fauna ... hopefully.
- Water travel should be at least known for its ability to carry goods more efficiently over water than land.
- I would expect that the tech is stalled as really only river and small lake travel would be needed in quantity.
- There could easily be a boom in shipbuilding triggered by advances if the condition is right
- There is likely an uneven split of resources in this newly split world
- There might be theories or inventions that were once regarded as novelties in the world before that become important now.
- A shift in the flora and fauna populations of the world will happen due to the changing of the lands -- how humans deal with that may be interesting.