There are two uC chips on the the Uno, the main 328p and the 8u2 (or similar) for the USB support.
You will almost certainly have fried both of them.
If you have a socketed 328p, you will be able to replace it with another. The smd 8u2 chip is not a practical replacement, though.
In this situation, if nothing else was damaged, you will get a Uno that can be programmed via the ISP pins, or via serial using an external USB->TTL serial converter. The USB connector on the board may be usable for powering the board, but nothing else. You may find the fancy power source auto switching circuit has gone out to lunch as well, though, as it employs an op-amp, which may or may not have survived (many op-amps can tolerate 12V no problem, though.)