Why the phone goes dark screen suddenly

@michelebolton if it is just the screen that turns black while using it, then check the proximity light sensor. Use the instructions from the Samsung site for that. If that does not resolve it, you may either have a bad sensor or a bad Sensor Flex Cable

If it is an issues with the adaptive brightness thn you go to settings>apps, then enable show system apps then search for a system app named "BrightnessBNR" and then clear that data. To find that system app make sure that you enable " "show system apps"