In 1385 DR, Mystra - the goddess of magic, managed and maintined the Weave, a matrix of power that fuelled all arcane power on Toril. Shar, the goddess of shadows, had long maintained a rivalry with the goddess of magic, dating back nearly to the dawn of time. After several failed attempts to replace The Weave with her own Shadow Weave, Shar sent the mad god Cyric to kill Mystra. Cyric suceeded on 29th Tarsakh of that year but the aftereffects were disasterous. The Weave could not exist without Mystra and magical energies broke their bonds as the Weave convulsed violently, causing the planes to shift; Deadly blue fire engulfed entire nations and masses of land floated into the sky, becoming earthmotes; The raw power unleshed mutated people and creatures alike, giving them strange magical abilities; Most arcane magic failed or backfired and many thousands of magic users were killed by their own abilities or descended into madness over the next decade.
The god of Knowledge, Deneir, sacrificed himself to try and stabilize the Weave, but the next ten years were a time of untold chaos. Only the most powerful of magic items and effects remained stable enough to use relatively safely so many began to rely on artifice and the power of their gods to survive the turbulent time.
Of course, not everyone suffered from the effects of the Spellplague, some even profited from it. Although nowhere was completely untouched by the problems caused, certain places fared much better than others.
Asmodeus, lord of the Nine Hells, captured Mystra's friend and servant Azuth, the god of wizardry and stole his divinity, becoming a god in his own right; Shar, although she lost the Shadow Weave along with the Weave, managed to combine the Plane of Shadow with the Negative Energy Plane, creating a realm more grand than of all the Astral Dominions that the rest of the gods now resided in - The Shadowfell - a dark land populated by undead creatures and Shar's loyal servants.
Eventually though, the surviving wizards found a new way to access magic that didn't draw upon the Weave and things began to settle down a little but the Spellplague was a dark event in Toril's history, that no one is likely to forget.