All was fine yesterday. Today, it’s not seeing the unit. What to check?
Could be a lot of things. I did this yesterday and was scratching my head only to realize I had not turned on the X Carve. It could also be your USB Cable or something loose between the G Shield and the Adruino Board. Good luck!
Unplugged cable, restarted browser, nothing. Restart coming next. I hate restarting, and, I even have an SSD. 
Update: Restart fixed it.