I'm running Windows 10 which I bought off of eBay for £2
It
might not be a legit copy of Windows but regardless the way to fix it should be the same.
If you can't get your PC to boot at the moment, best thing to do would be to switch the power off at the wall or unplug it from your extension lead. With the power off, hold down the PCs power button for about 10 sec.
Then power back up and boot, it
should work.
To stop it happening again:
Right click Start Menu > Settings > System > Power and Sleep (on the left hand side)
Then click additional power settings (right hand side)
Choose the option 'Change what the power buttons do' (left hand side)
You may need to select the blue 'Change settings that are currently unavailable'
Under Shut Down settings, UNTICK 'Turn on fast startup'
To finish, click Save settings then reboot your PC.
That should, hopefully, stop this happening - if it doesn't let us know as there will be a few more steps to try.