Unbricking
If after flashing the firmware and rebooting, you've waited a good 60s-90s and your device won't boot, you may need to unbrick it - a process which involves re-flashing the firmware using an external programmer.
TIP
If you think your device is bricked, the quickest and easist thing you can do is to perform a Hard Reset (AKA an EC Reset): press the [Refresh+Power] keys simultanenously, then release.
There are serveral methods which can be used, though not all apply to all devices. All 2017+ models with a CR50 Google Security Chip (GSC) can be recovered with a SuzyQable.