System reset:  Unplug the system, disconnect all external devices, and remove the base cover.  Disconnect the battery from the system board and hold the power button for 30 sec.  Reconnect the battery, replace the cover and power up, holding the D key.  A series of self-test screens should show -- if they do, then power off and reconnect the external devices/monitor. If you do not see anything on the screen at all, even under bright light, contact Dell to have the display replaced under warranty.  If you do see a faint set of images, it's more likely the power to the backlight is the issue and the mainboard needs replacement (which the warranty will cover).  
I have a recently purchased Dell Latitude 3520 with a display issue, and I hope someone has a solution.  It's running Windows 10. It has an external monitor plugged in, and it was using a dual display setup for a while until the screen on the notebook just stopped working today.  The external monitor works fine, but the notebook screen is totally black, and not even detected as a second monitor when I look at the Display Settings. The really odd thing is, I set up a remote connection to it as a test, and when connected remotely to the notebook, and the external monitor is unplugged, I can still use the system fine from the remote connection, but the actual screen is still black. The Fn+F8 key on the keyboard was tried too in order to toggle the external display, but the notebook screen is still black. Any suggestions?  
Dell Latitude 3520 black screen, but external monitor works