XPS 15 (9559) dos not start

I'm not sure it's linked but last week I performed a regular laptop update, among others, Dell wanted me to update the thunderbolt (if I remember correctly).

In any case, since end of last week, the computer doesn't start anymore.  In fact, when I press the power button, the LED behind the button lights up, sometimes the fans starts but nothing whatsoever is seen on the screen and after a while, the computer automatically switch off the power again. 

Powering up the computer while oressing the D button results in a screen flickering in different colours which I think is correct.

The other simple test  I've performed is to press the Fn togeter with the power button but this doesn't lead to anything.  Other things I've tried is to press the power button for a long time, taken out the battery and disconnected the BIOS battery but nothing helped.  An attempt to start with a boot disc (USB memory) didn't work either although it was powered.

So, what can be done now? To me, I've a suspicion that something went wrong when the BIOS was flashed with the Dell update last week.  NB, the computer was powered all the time during this process.  For information, normally the laptop is connected to monitors, keyboard etc through a Dell docking station.  The tests above have been performed with the laptop disconnected from the docing station.