The Shattering is an event that occurred at Year 0. Nobody knows for certain what happened, but one moment there were countless planes, endless worlds, and a multitude of gods; in the next instant the sound of shattering glass and tearing fabric echoed across the realms, and in its wake everything changed. The gods were decimated, entire worlds were slammed together, the gaps between planes became nigh-impossible to cross... and life continued.
Thousands of possibilities have been suggested as the source of The Shattering; did Sigil, the City of Doors, suffer a dimensional accident that echoed through reality? Did the gods wage war, with each other or with the devils of the Abyss? Did time reach its natural end and reset? Did the Weave cave in on itself and bend the universe as it failed? Did the cult of some dead deity try and fail to bring them back... or did they succeed, even? Did the Host personalities cause the Shattering or did they come into existence because of it? Truly, nobody knows. If anybody might one day find out, it would be the Archmage or the Priestess, but this writer doesn't know for sure that the cause of The Shattering will ever come to light.
In the instant The Shattering occurred, a demiplane called Ruin came into existence. Areas of the Material Plane with doors open in Sigil were wrenched from their vanishing worlds and dashed together before Sigil's doors closed for the last time. Small areas of Faerun, Eberron, Krynn, Athas, and Oerth, and Ravenloft in its entirety found themselves co-existing, with only a light ethereal mist representing their borders. Sigil is visible in the centre of the demiplane, but conventional attempts to breach its walls have been met with an impenetrable barrier. From close by and with adequate equipment, onlookers have been able to see life continuing as normal on Sigil's ring, but have no way of knowing if this visibility works both ways.
Links to other realms were completely severed. The rest of the Material Plane is completely inaccessible, while the Feywild and Shadowfell have adapted to match their new reflections in Ruin, albeit with adventurers finding travel more complicated in this new world. Similarly, the Underdark has adapted and serves as a route to access the Elemental Chaos without the danger of planar travel, and from there one can travel to The Abyss and The Nine Hells. With an airship, one might possibly access the Astral Sea in search of the other worlds and the gods, but such excursions would be fraught with danger and uncertainty.
Magic briefly ceased to function in the months after The Shattering, which was especially problematic for Sharn, Zilpar, Tamthus, and Reven-- the dwellings previously hailing from Eberron, as their magitech relied on magic at its very core. After the thirteen Hosts appeared, magic flowed back into the world once more, or for the first time, depending on how one views it.
The Hosts themselves speak of Ruin as if it is the only remaining Material Plane.