It’s rare, but it can happen. Sometimes the Chromebook hardware can lock into an unknown state that essentially looks like it has stopped booting or appears to be out of battery.
In this case, when you can’t get to the options for restarting or resetting the OS, you can perform a hardware reset.
For most Chromebooks you perform a hardware reset by pressing the Refresh and Power buttons simultaneously.
Some Chromebooks have an actual power button included in the keyboard layout, but for those without, you’ll need to use the hardware power button on the Chromebook’s side with the Refresh key.
Additionally, for some Chromebooks, resetting the hardware requires pressing a special reset button, unplugging the power cable, or removing and re-inserting the battery. Search your specific manufacturer’s website for the proper sequence.