Very new Arduino user needing MAJOR HELP ASAP

Hello guys, i been 3d printing for the past couple months making PPE stuff for the local palces that will use them and was told if leaving my Ender 3 pro unattended that updating the firmware will enable thermal runaway protection. so i tried it on my Perfectly working Ender3 and can only get so far after burning the bootloader onto it which i know worked cause the IDE said so and now i have no firmware. i did this exact same process on my P802ma(Prusa i3 copy) without any issues. The issue im having is this.

core.a(main.cpp.o): In function 'main':

C:\Program files (x86)\Arduino\hardware\sanguino\cores\arduino/main.cpp: 11: undefined reference to 'setup"

then the exact same as above except .cpp: 14: undefined reference to 'loop'

sorry if that the dotes are in the wrong places and ANY OTHER INFO just ask i will provide it all

PLEASE help me i need my printer working again BADLY, first person that get me fixed gets a custom 3d printed key chain with there name on it sent to them free i need to be working again very badly,

Thanks

A valid Arduino sketch (program) must have a setup() and a loop() function even if they are empty.