The board uses either an ATmega16U2 or an ATmega8U2 for USB-to-serial conversion.
The processor supports "dfu" mode for reprogramming. You may be able to restore the processor using that mode. This should get you started...
https://www.google.com/search?q=atmega16u2+dfu+restore
If that doesn't work, you can use an "ISP" programmer (another Arduino compatible board will work) to try to restore the processor. This should get you started...
https://www.google.com/search?q=arduino+atmega16u2+isp
I have not done it myself so that will very likely be the only guidance I can provide.