Well then, if your computer is completely shut off, and when you turn it on it only shows black screen, then you have another problem , not resuming from sleep.
Sleep state saves your current state to RAM, which means that a power shutdown clears this state and you should be able to boot from scratch.
If you hibernated instead of sleeping, then again you should see at least the BIOS welcome screen when you turn on your pc, and then windows should try to resume.
But if it's all black, I suggest you check for connection problems, monitor cables disconnected etc. I know you will say it worked fine before you set it to sleep, but believe me, you never know. If the problem persists, you must consider clearing the BIOS from the motherboard jumper or changing a PSU, in my technical experience.
Tip: Leave the computer without power cable for at least one minute, then try again.