well, what did you do to it?
you did something.
is it a genuine samsung phone?
if so, you need a computer, usb cable and you need youtube.
the oficial firmware is around 1GB.
so go to youtube.com, and search for "update galaxy s3 firmware"
the video wil give you instructions and download links.
no matter what you do to your phone, the bin will always be intact and you can always re-awaken your phone using Odin. that includes your Macdonald. you can get it up and running