This may be where you wish you had done "File -> Export UAV..." when it was working.
You can erase settings like this:
- unplug battery and USB
- on firmware page press Rescue
- plug in board USB and wait a few seconds
- Erase button should become active, press it and follow directions
- Start over
If you know what you did to break it, then instead of pressing Erase, press Safe Boot, then go to what broke it, make a change (required) and Save. Reboot the board.
I bet you changed something on the Hardware page fiddling with telemetry, so instead of pressing Erase go to the Hardware page (it will be all defaults), make a simple change, Save, reboot, see if it will connect now.