Laptop turns on and off in seconds and repetitively?

My laptop suddenly won't turn on. It is only on for a second, then off again, then on again, etc. Without my pressing the power button.

Have already tried hard reset.

If you speak of a second then unfortunately there can be a lot behind it. But what you can check first is the power adapter because it sometimes causes problems like this.

Then it is probably defective and you have to have it repaired.

Sounds like problems with the power supply

As the others already write: This is a hardware defect.
Whether it is a broken cable / loose connection in the power cable, whether the battery is simply through or what is serious can't be found out from the description.

A technician should take a closer look at that.

The power supply works. I tried it on another laptop. Laptop does this with and without the power adapter plugged in.

Take out the battery once and wait about 20 minutes before trying again.

Great, you've already ruled out a factor.