I couldn't really find an article for your scenario, but here's what info I collected:
Common sense, just keep your laptop/PC in a safe place and make sure it has absolutely no power. Don't mess around with it until you can figure it out.
It could be the outlet you're using.
The USB cable inside the PC could've shorted and touched another wire, causing a spark and then, of course, the smoke.
The cable you put into the USB port could've had some kind of material or liquid that doesn't really react well with electricity (e.g. water)
I've never really experienced too much with this kind of thing, except for Xbox 360 transformers short-circuiting in another country with poorer electricity. I'm just listing some causes I found from quickly searching around. Also, what did you plug into the PC? Where was it before? And lastly, [b]You should probably send this to some PC repair shop to fix it up. Don't try to fix it yourself, and don't power it on.[/b] I hope I helped.
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It was the cable from USB to micro USB. The cable was in a dry place, so water couldn't have done it...