|Whether your computer attempts to turn on automatically can depend on settings in the mboard's bios Setup.|
E.g.- if it is set to power on when AC power is restored, but that's not a default setting.
Some desktop mboard bios versions cause the computer to start up briefly when the power supply receives AC power after that having having been off, regardless of settings in the mboard's bios Setup, but then the mboard shuts off, if there's nothing wrong.
If that applies to your mboard, that would have happened when the computer was working fine.
The computer does not have a Restart button, but if it's a desktop computer it might have a Reset button.
If the operating system is not working properly, or if there is some hardware problem, pressing the Reset button often does not work.
However, also depending on a setting in the mboard's bios Setup, usually, by default, holding the Power button in (or down) for at least 4 seconds shuts off the mboard.
You have not provided enough information.
People often think there is something wrong with their motherboard or the cpu (processor) on it when their computer won't work properly, but in most cases there's nothing wrong with those - usually something else is causing the problem.
What is the make and model of the brand name computer, or if you have a generic desktop computer, what is the make and model of the motherboard ?
Was this same computer working fine previously ?
If yes, have you just made any changes to it , or have you been fiddling around with anything inside the case if it's a desktop computer, wiring connections or components, before your present situation ?
Did you have any symptoms of something being wrong for a short time, or for a while, before this present situation ?
If so, describe them, in as much detail as you can come up with.