If there were some cataclysmic effect such as the realm passing into another plane of existence there is no reason to expect that mortals would even notice it and would just continue on as if the new circumstances had always been in effect. This has been a common construct in fiction that has dealt in these matters. Certainly the protagonist comes to the realization of the change but there is no reason to expect that you are he. Often even his realization is lost after a time.
