No reaction on power on

[KlausL3]

No reaction when I turn on the laptop

Notebook Spectre x360 from 2015. The notebook was connected to the power supply for about a year, but was never turned on.
When I now turn on the notebook there is absolutely no response: no LED, no fan, no screen, no sound.
- The power supply is fine; it works on another laptop.
- I have opened the unit and unplugged the battery, but that doesn't change anything.
- Somewhere I saw to press Windows key + B + PowerOn: no effect.
Is there anything else I can do without sending the laptop in for repair?

[WAWood]

@KlausL3 

Sorry for the bad news, but basically, no.

If it is NOT possible to run diagnostics, or if that does not work, then there is NOTHING more you can do by yourself -- and there is NOTHING we can do because we have no way of accessing your PC from here to run diagnostics or repair hardware.

[KlausL3]

@WAWood

Thank you very much for your reply.

I called HP Support today and after some diagnostics they explained, that there is no repair possible. They assume, that a powerboard (I'm not sure they called it that way) or the mainboard is damaged and there are NO MORE SPAREPARTS for this 5 year old model. HP doesn't want that I send the laptop for repair! Sure you can imagine how disappointed I am.

On the other hand, I'm getting braver and ready to do more on the machine myself. One glimmer of hope is the following symptom:
- The power supply may not be connected for a some time.
- Press and release the power switch for 15 seconds.
- Connect the power supply.
Now the small white LED next to the power supply socket flashes three times.
Can anyone tell what this indicates?
As a reminder: my power supply is okay (works on an EliteBook) and the battery was tentatively disconnected with no effect.

The flashing of the LED is only repeatable after disconnecting the power supply again for several hours.

Any help gladly welcome!

Klaus