Hey!
I just wrote this:
https://forum.arduino.cc/index.php?topic=506341.new#new
You may have to do all the process, but its worth checking Nick Gammon's tutorial on how to check if the board is fine first!
If your board is fine, you should do what I wrote!
Nick Gammon's tutorial has everything you need, from checking the bootloader, to recovering it!
Check it out!