We have a basic tech doc on this: http://docs.carbide3d.com/software-faq/can-t-connect-to-machine-or-jog/
To expand on that, things to try:
- re-flashing: GRBL 1.1 and Carbide Motion 4
- clearing the EEPROM: https://www.shapeoko.com/wiki/index.php/Grbl#Clearing_settings
- re-loading the bootloader using a programmer/second Arduino
If that doesn’t work, new board is most straight-forward.