Inspiron 24-3455, black screen

Did somebody accidentally press button #16 (Screen‑off button) here, and turn the onboard display off?

EDIT: You may have to press #16 more than once, because that button may "cycle" options between On-Off-external HDMI.  So press it, wait a few sec and if that doesn't help, press it again and wait...

Yes I e tried this that’s what lead to the external monitor usage

Reboot and immediately press F2 or F12. Do you see anything on the screen after pressing either of those keys?  If you can see the screen after pressing F12, look for the option to run diagnostics and run them.

Boot the PC and take a bright flashlight and shine it at the black LCD panel from an angle (not straight on). Look closely and if you can see things on the screen, then the backlight in the LCD panel may have failed and the screen will have to be replaced.

Do you have a bootable USB stick or DVD? See what happens with the screen if you can boot from one of those.

Beyond that, it's virtually impossible to say if this is a simple cable failure, an LCD panel failure or some other failure. So if the PC is still under warranty, contact Dell Tech Support ASAP. Have your Service Tag handy when you contact them. Otherwise, you may want to take it to a reputable PC shop for diagnosis and a repair estimate.

NOTE: You may have to try those tests a couple of times after pressing the #16 button in between, because you probably don't know if that button currently has the internal LCD set to on or off...

I occasionally experience the same problem; in the future this will be fixed.  What I believe is happening is a static charge builds up on the touch screen.  This has happened to a tablet of mine too. My tablet has a screw under a small panel that I removed, which released the static charge and then the touch pad became responsive again. There is youtube videos of people removing the display connector to achieve the same result.

   For the Dell Inspiron 3455 all you have to do is hold the power button for 6 seconds to turn it off, THEN unplug the power cord from the back of the device for at least 10 long seconds to allow the static charge to dissipate and your touch screen should be good again. It is a safety feature...  Works for mine. 

Here is a little FYI for y'all, did you know that when you shock someone or something with your finger and you can see the shock, that means there is at least 3,000 volts being transferred! Computers run on 3.3 volts, 5 volts and 12 volts, that is why electro-static discharge from US touching certain components fry's them.  Such as, Ram and SD cards... 

I bet that is what I did I had no idea it had a screen off button above the power button,seems like a strange place for that ,IMO they should have put a small switch by the HDMI port