Looks like one the 2 issues:

1. The display has died, it's okay as long as the motherboard is fine, which in this case, motherboard is working, because external display is working fine.

2. Ribbon cable connection of the display to motherboard has broken or is loose.

If you're comfortable with hardware and confident enough, just open the back of the macbook and see the display cable carefully. Atleast you'll be able to diagnose the issue.

Did you try increasing the brightness?

I did a screen replacement on one of those. I don't recommend it if you have little or no experience especially.

screen replacements are REALLY expensive, get a very cheap mid-2012 MacBook Pro as a backup option for €100-€200 ish

plug it into another mac and it will work if its not the screen.

Caps key led toggle on/off? If yes, then yes, the unit is booting. External HDMI monitor is not working? Test with another known good USB-C to HDMI dongle - select one with solid ratings and hopefully are not fake reviews. Amazon is the fall back plan but they do sell a lot of junk and non-compliant products. Sounds to me that your lid angle sensor is not working properly and unit needs to be calibrated. If there was no abuse to the unit, the backlight should work ok. Essentially, when the lid is closed, the backlight is requested to power off to save on energy / battery drain. At this time, the sensor believes the lid is closed. That is my quick take on the situation.

Seems like it's getting power and turning on but there's no video. If anything, it would seem more like it needs a display than a logic board.

But, if it's a 2018 or later model, you can also try to revive the firmware. Sometimes that fixes video or power issues.

It's certainly a hardware fault. I could be the display or it could be the GPU/mainboard.

If no video outputs to an external display, the problem is most likely the Main Logic board.

A first step would be to find out if it's the screen itself that's screwed up, or something else preventing it from starting.

Power it on and leave it for a couple minutes. Then, check:

1. Caps lock key light
2. Changing keyboard brightness
3. Changing screen brightness
4. Holding shift and tapping the volume up key -- does it make a sound?

That'll help you figure out if it's a simple screen replacement, or something more along the lines of RAM, logic board, etc.

Already tried to boot into the recovery mode?

Hi there, I know this is an old post, but this has happened to me, many times before. Please read this entire comment before taking any action.

So heres my story: one day I open up my very expensive and very new macbook pro (2015) (yes I know this comment is posted in 2018 but the first time I encountered this problem was late 2016 or early 2017). Shockingly, the screen was completely black and not lighting up WHATSOEVER! everything else was functioning, lights on the keyboard, sound on the computer, etc. I tried everything, like you. I tried resetting RAM and all that other stuff. I tried booting in safe mode. I literally tried everything thats suggested on the internet, whether through apple or other sites (such as reddit).

Heres what I have to say: there is only ONE solution that actually solved my problem. But before I tell you, I must say this, this solution that DID WORK FOR ME AND YES IT SOLVED MY PROBLEM, will void any warranty or applecare insurance that you may have. So if you have applecare or a factory warranty or ANYTHING like that, please take your computer into a store and have a licensed professional look at your product. Anyways, I do not have applecare or any factory warranty or anything like that. But heres how I solved my problem: i took the back off of my computer (MacBook Pro 2015, 13 inch) and disconnected the battery. Leave it disconnected for about 1 minute or so, then reconnect the battery.

Look, I cant guarantee this will solve your problem, but it solved my problem. Literally NOTHING else on the internet would work. But somewhere down the line I read about disconnecting the battery from the computer, and it worked for me. This is literally the only thing that solved my problem. After talking to an apple "professional," I was told that the issues was with the logic board. After only 1.5 years, my computers logic board failed! What a surprise. But in a nutshell, the way I solved my problem was simply disconnecting the battery and reconnecting it. I've had this problem about 3 times in the last 2 or 3 years and the solution I provided has worked every time.

Again, I cant guarantee that this will solve your problem, but if all else fails, give this a try (but again, do not do this if you have a factory warranty or insurance because disconnecting the battery or even removing the back cover will likely void whatever warranty/insurance you have).

EDIT: I've tried restarting in recovery mode as well, DOESNT WORK. Using an external monitor DOES WORK. I've connected an HDMI cable from my laptop to my TV and yes, it does work. I get the screen from my computer on my TV. but that's only a temporary solution. Again, literally the only solution I found on the ENTIRE INTERNET was disconnecting the battery, leaving it for about a minute or 2, then reconnecting it. Again, this will void whatever warranty you may have, but with me, I didnt have any warranty, so this was better than paying hundreds of dollars to Apple to fix a hardware problem that my dad helped me fix in 10 minutes ;). Good luck man, hopefully you read this comment before spending hundreds on Apple to fix your issue. I TRUELY HOPE my comment can help at least one person out there. Because I know what it's like to drop about $1000 on a new laptop and not have the funds to have it repaired, so I hope what I said can help someone out there in need.

Can you use an external monitor?

Just press the power for a while for force restart? I seems to encounter the same problem and solved it by force restart

sounds like backlight is not lighting up. Might be a simple fix in a normal repair shop. Apple service might ask you to buy new pc so try in a well rated repair shop

Take it to the genius bar if there's one nearby and you should be able to get it fixed up for free if there's no water damage and you didn't crack the screen physically.

Bro there is no work around I just came here to ask for help.

I gave my lap to service centre two days back and man they're charging literally 50% of what was the amount I paid to buy this. And they are stating it's display issue while I'm sure it's not cuz have spoken to lot of people here who owns M chip old ones

I think they're phasing out older ones I believe. Don't fall for the hardware issue

Bro same thing just happened to my M1 Air. I was watching YouTube and then I closed the lid normally and after half an hour I opened it and the screen was blank except only backlight is there and all the buttons are working normally. I tried many troubleshooting but nothing is working for me

you have just performed "half/split" troubleshooting.

now you know the problem lies between the display cable connection and computer.

the cable could simply be loose, cracked or the display has gone bad.

i'm not familiar enough with this model to know what is most common.

macbooks usually need someone familiar to service .

the cable is $20, the screen assembly is $450 from ifixit dot com .