You can use a hardware technique to get your Glass into fastboot. Make sure your device has some charge (15% or more should be sufficient) and follow these steps.
- Power down your device by holding the power button for 15 seconds.
- Press and hold the camera button. Keep holding it until step 5.
- Briefly press the power button.
- Wait until the LED solidly illuminates.
- Release the camera button.
Now your device should be visible to fastboot. Plug it into a USB port and use fastboot to list devices:
$ fastboot devices
If your device is listed, you can now run other fastboot commands.