This is a really peculiar case that I have, so I'm going to write out as many details as I could.
I use an asus laptop with graphic card nvidia 960m. I installed windows 10 in my ssd drive and ubuntu 18.04 in hdd, using grub to manage my booting options.
When I use windows 10 everything works fine. If I boot into ubuntu, use it and shut it down, then the next time I press the power button, the power button would light up, and the fans would start to rotate, but the screen wouldn't show up anything, not even asus logo, let alone the grub. The same thing will happen again even if I force shutdown and press the power button again. I can't even get into bios.
The temporary solution I found is very strange: When I shutdown ubuntu, I close my screen lid and open it up, then press the power button. This blank screen issue would be gone and the grub will show up, then after choosing the system I want to use I could boot successfully... this time. I have to force shutdown and perform the rather stupid solution every time I get this blank screen, but it just doesn't happen when I shutdown from windows.
I have tried to purge and reinstall grub, purge and install nvidia driver 435, use nouveau.modeset=0, boot-repair. I still couldn't solve the problem. So I'm not sure where the problem is. It might not be the graphic card driver that cause the problem at all, judging from the simple solution I found. Then again I couldn't find the cause. The hardware might not be the problem because when I shutdown from windows 10 I wouldn't get blank screen the next time I press the power button.
I hope there's someone who can identify the problem. Thanks for reading this long post.