Thank you for your help: I could see the bios again with an HDMI to HDMI cable. Then I fixed my OS with a bootable USB stick. If I plug the NUC with an HDMI to DVI cable, then I can no more see the boot sequence... but my OS is working. I guess I had a double problem.
The NUC is the only computer I have not displaying properly the boot sequence if using HDMI to DVI cable. I hope future versions wont have this issue.
loving this Bay Trail so far !
Thank you for your support.