Hey, see here for steps how to reflash the bootloader: USB Device Not Recognised - #9 by Giedrius - an external programming device is necessary, such as an Arduino UNO.
The bootloader hex is available here: Blokas-Boards/avr/bootloaders/Midiboy-Bootloader at master · BlokasLabs/Blokas-Boards · GitHub
However, from what you describe, it’s probably not an issue with the bootloader, but rather the screen. Check if it has good contacts and that the ATmega328P IC is installed well.