oftentimes (and this is more of a general statement) throwing into google exactly what you would otherwise type into your shell of choice should get you on the right track, ie searching for "man systemctl"

as far as the inability to reboot goes, if a regular sudo reboot can't bring the machine back up either then this is probably a hardware issue outside the sphere of the operating system's influence. can't say I experienced something like that myself. I guess the closest I witnessed would be a computer that when rebooted with an old USB-Keyboard plugged in just refused to get past the POST screen. The keyboard worked fine if plugged in later, but the computer couldn't reliably get through the boot process with the thing present. Maybe there's a similar variable to your setup.