For aeons, most magic was practiced via something known as The Weave, which was maintaned by the goddess Mystra. About 50 years ago, Mystra was murdered by another god, put up to it by Shar, who was hoping to take over The Weave, despite having her own version, The Shadow Weave.
The result of Mystra’s death was that The Weave came unraveled, as did the Shadow Weave. This had very serious consequences for Faerun. In many areas there was Spellplague, which destroyed terrain, and strangely transformed some of the creatures there. Great portions of The Underdark collapsed, leaving craters on the surface, sometimes thousands of miles wide. Many of the magic portals which had been operating ceased to operate. And there were motes of land left floating in the air in some areas, suspended by no obvious means, but stable nonetheless.
After a time it became apparent that there were whole new geographic features, cities and civilizations now on Faerun. It appears that an alternate plane, bearing a close, but not exact resemblance has been “merged” with ours. The consequences of that are still being worked out.
Finally, of course, all magic had to be relearned. These days, basic spellcasting is quite different than what it used to be. There are still the same elements, but many things now take the form of rituals rather than spells. Often, access to basic magical effects is much easier, though the complex stuff still requires years of study and experience.