Why Is MBP Screen Black With Power Symbol But Not Turning On?

Hi there, actually, I've got the same trouble on my MacBook Pro a little while ago. The power cable was connected to my MBP but I couldn't turn it on, even after several hours. I changed the power cable to a Lightning cable and the power charger to a MagSafe adapter, then my MacBook Pro turned on successfully. So you're recommended to check if your Mac is powered properly. If there is nothing with the power supply, then you should try to reset the SMC to get this trouble fixed.

On top of resetting the SMC, there are alternative solutions that are worth a shot as well. You can have a look real quick here and try one or more as you like:

Solution 1: Detach all external devices from your Mac

Solution 2: Let your MacBook charge for a longer time

Solution 3: Perform a power cycle on your computer

Solution 4: Check the display brightness settings or try to use an external monitor

Solution 5: Seek a repair service

Hope this answer helps the Mac won't turn on trouble that you encountered. Good luck~