MacBook only works with external monitor – built-in display stays black, keyboard not working

My MacBook (2021m 16 inch, Apple M1 Max, 32Gb, 1TB, Sequoia 15.6) still works, but only when I connect it to an external monitor. In that case, I can use it normally via the monitor. However, the built-in keyboard doesn’t work (it lights up, and the power button works), and the internal display stays black.

Here’s exactly what happens now:

• When connected to an external monitor and turned on, I briefly see the loading screen on the MacBook’s built-in display, but then it goes black.
• On the external monitor, the MacBook works fine.
• If I try to turn it on without an external monitor connected, it doesn’t start — the display stays black.
• The keyboard doesn’t respond at all, but it does light up, and the power button can turn the MacBook on or off.

What happened before this issue:

A while ago, I had my MacBook sitting next to the sofa, and it suddenly started repeatedly turning itself on and off without me touching it.

After that, I tried to start it normally — it would show the loading bar but then stop and shut off.

At one point, I attempted a reset (PRAM/SMC reset), and I briefly saw the reset-related window on the display once. However, since then, that screen has never appeared again.

Now, the only way I can get the MacBook to start is by connecting it to an external monitor.

Question:

Has anyone experienced something similar? Could this be a macOS/software issue, or is it more likely hardware-related (e.g., display connector, logic board)?

Any advice on how to troubleshoot or fix this would be greatly appreciated.