You've been grabbed by a vortex and taken to a new Earth, no on knows why or how.
Apparently there are an infinite number of Earths, and many of us are from entirely different worlds from your own.
Here, we have this forum, where we talk to others like us. And we have portkeys, magical objects disguised as normal everyday things that allow us to teleport between cities.
No one knows how to go back to their original world, no on has managed to do so of their own will.
If you look in your wallet, or somewhere on you, you should find identification that will tell you what sort of person you replaced, or at least an address. When we come here, we replace someone and to everyone else, it is as if we always were doing whatever they did.