I recently came across a fix for a Dell XPS15 9560 (late 2018 model) which was constantly restarting as soon as the power was switched on, it wouldn’t even reach the BIOS screen and pressing F2, DEL, F12 on startup did nothing. The keyboard lights were just flashing on and off and it kept restarting. This can happen if your UEFI secure boot gets messed up and the laptop can no longer access any of the recovery images.
The simple fix was hold the power button for more than 25 seconds but less than 40 seconds – then release. i.e. hold for 30 seconds then release.
The computer will reimage the BIOS (the keyboard lights and LED at the front of the touchpad will flash a few more times).
After this it will boot up and say there’s an issue with the clock setting – just hit continue. Then it should boot normally and be back up and running.