This informatation came from:
HOWTO: reflash bootloader to Mega2560 w/JTAGice mkII and Atmel Studio (https://www.avrfreaks.net/forum/howto-reflash-bootloader-mega2560-wjtagice-mkii-and-atmel-studio)
OK, techically..., you can use any programmer capable of programming via the ISCP port (6-pin block) near the center of the board. I used an Pololu USB AVR Programmer v2.1 (https://www.pololu.com/product/3172). I put it here so I could find it again. And perhaps if you got here you found what you were looking for.
Symptoms: