Sometimes it may happen that your machine gets stuck. A message like “The system is going down for reboot NOW” stays as the last line on the command prompt when you asked for a restart, and there is nothing you can do. Or some other times, your machine has gone into sleep and doesn't wake up, no matter what you try to do to bring it back from Morpheus' arms…

When it is that serious, power cycling the machine may be the only option, but it can leave some files in a state that will hinder the restart process.

Even if, at restart, your bootloader shows the startup options and it brings you to the GRUB bootloader selection. All might seem well, until the screen turns black with the Linux Mint logo, only a few dots blinks before they stop… Waiting a few minutes won't do, nothing happens: power cycle again !

If you select the “recovery mode” in the GRUB menu, so you could see the command line output, it eventually will get stuck again after a few seconds (I went through this many times to make sure it wasn't a temporary problem :-/)

From previous experience, I discovered that, sometimes, after a system crash and a power cycle restart was forced, turning off the computer - unplugging it - removing the battery - keeping the power button down for a few seconds (on a MacBook laptop) - replugging everything and starting it again did the trick!
I still don't know exactly what is going on at that level, but I guess that involves some sort of PRAM information or the like, that prevents the computer from starting up correctly. Maybe I'll get some time to dig that matter someday.

Well, after all that “hardware reset” stuff, everything was back on track and startup went on as expected…