Did it just shut down for no reason at all? That's odd, is it broken somehow?
When talk about fastboot, you are refering to the screen that shows up when you press vol up + power from a power off state, correct (where it shows a red warning triangle with "Fastboot Mode" below it)?
From there, pressing vol down to choose "Recovery mode" then pressing power shows the "your device is running an alternative operating system", but then just shuts down, instead of showing the recovery screen with the text "No command", right?