Hidden fastboot mode or no fastboot at all?
To go in Fastboot mode: 
- Switch off the phone
- Hold Volume down button
- While holding down the button, connect the USB cable to the phone (assuming the cable is already connected to the computer)
The phone should go in the Fastboot mode. 
In the command prompt, tape fastboot devices , you should see xxxxxxxxxxx device as feedback.
Drivers must be installed as well.
Hope this will help.