Something has gone horribly wrong in Mundus, a fairly standard medieval fantasy world. A rather eccentric wizard was carrying out some research and one of his spells blew up in his face, causing a catastrophic event: throughout the world, every sentient being--humans, elves, dwarves, dragons, even the Gods--suddenly vanishes all at once, caught up in the wild magic and instantaneously carried forward in time by approximately 300 years.
When they emerge, after a brief but intense period of vertigo, they find they're in the same place as they had been, their clothing and anything they had on their person is still intact, but the rest of the world has lived through those 3 centuries the normal way! Non-sentient life (plants, animals, microbes, etc) was not affected (directly) by the spell.
What kind of challenges do they face, trying to re-establish society? They would not have as difficult a time as we would today, as there was less advanced technology to be dependent on, but all of their cities and dwellings would be in ruins to one degree or another by this point.
Note: Just to keep things simple, at the time of the Catastrophe, no one in the world possessed any industrialization or large-scale magic capable of causing significant changes to long-term climate patterns.