XPS L502x does not boot anymore after BIOS update to A12

Hi there,

I've updated the BIOS from A10 to version A12 . I've used the method "Run the BIOS update utility from Windows environment"

The updated seems to have been installed without any errors. (no errors appearing, installation went to the full 100% and I've got the message the system was going to reboot after, 5, 4, 3, 2, 1...)

BUT...It did not reboot, so after 15 seconds I hit the start button. The lights blink in the following order

 power (stays on)
wifi, battery, settings, share, audio, share, settings,battery and wifi again.

after that the computer shuts down again. Every time...

So it seems the BIOS was not successful after all!

power+fn+B button combination does not work
power+D button combination starts the screen test cycle.

I already reinserted the RAM modules and the HD. 

My dell is not in warranty anymore. What can I try?