I've had this problem since I purchased my All-In-One HP Pavillion with Windows 10 pre-installed and it's been bothering me even since.
I will sometimes set the computer on "Sleep Mode" and when I come back to it, it won't restart by pushing any key or by any movement by the mouse. The only solution is to reboot from the power button!! The same happens if I leave the computer to pick up a call etc and come back to it...same black screen (it's gone into sleep mde by itself?) and I have to reboot.
This is quite annoying and also wastes a lot of my time. Please help!