Fixed this! I had the same issue with my laptop - on Sleep or Hibernate, when trying to wake it up, the Dell logo and loading animation (Hibernate) would run through and stop on a black screen. Found out that the computer was interactable in this state (tapped one of the links on the lock screen, and it opened in Edge after I woke the screen with temp. fix below), so Windows was loading, just not the screen. Temporary fix was holding down the power button for several seconds, which woke up the screen when it told me, "Swipe down to turn off PC" or something similar. Denying the shutdown let me use the laptop as normal. Anyway, latest, problematic driver did not reinstall after reboot when replaced with next oldest
I just got the new XPS 15 9510 today.  When the computer goes to "sleep" the screen will not come back on. It actually comes on very faintly, but it's so dark you cannot read or see anything except the outline of windows that are open. It's basically completely black. Nothing works to turn it back on: adjust brightness, hitting mouse, keyboard or power button.  The only way to get the screen back is to do a hard reset by holding the power button down for 10 seconds to reboot. Then it boots up and looks normal.  However, if the lid is closed, or the computer is inactive for a period (currently set for 30 minutes, but I have tried adjusting to different times), the computer enters sleep and the screen is black again.  I have already spoken to a technician who spent 90 minutes on the phone with me.  We updated the BIOS firmware, installed all the latest windows and dell updates. Updated the Nvidia graphics card drivers.  And then when none of that worked, we did a full system reset back to factory settings.   The problems persists.  I cannot have a laptop that is unable to enter sleep mode without forcing a hard reboot. Any ideas?  I'm not sure if this is a Windows problem, an Nvidia problem, or a hardware problem. Thanks!
New Dell XPS 15 9510 - Screen wont come on after sleep