Burn the bootloader from another Arduino

You can also use a 2nd Arduino to flash the bootloader. For example if you can't access the first Arduino for some reason.

Follow the instructions from the Installation sketch.

Just connect all wires from the 2nd Arduinos main MCU to the 16u2. You also need to connect 5V-5V and GND-GND to power the board. Unplug the USB cable of the Arduino you are flashing. In this case you do not need the capacitor.

You can also use another Arduino as ISP.