Generally you could potentially cause damage to hard drives if they are currently in use, but there are lots of things in place to prevent possible damage.
The bigger concern would be why your PC screen went black in the first place.
Was playing ffxv demo when i change to borderless the screen goes black and nothing i can do to minimize it, can still hear sounds but thats all.. already unintall that game, hope everythig is ok now, ty very much for replying