It is quite likely that you have caused some serious damage in either the USB charger or whatever it was charging (or both) by applying 12 volts to a circuit designed for 5 volts. If you are lucky, there could be a crowbar regulator in the circuit. This is a device that protects against overvoltage by blowing the fuse.
Most semiconductors fail short circuit when overstressed, so it is quite likely you have welded the layers of some semiconductor device. As I cant see the schematics of the charger and whatever you were charging, I can only make guesses.
To operate a 5 volt device, you need a 5 volt power source. It probably draws about a half an amp.
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