You should be able to load the bootloader through the ICSP header using an external programmer.
I have used these tutorials for the UNO, but you will need to get the correct bootloader for the Leonardo:
Using the Arduino software - Burning a Bootloader to the Arduino Uno in Linux using the AVRISP mkII
Using Atmel Studio - Burning the Bootloader to an Arduino Uno using Atmel Studio and the AVRISP mkII