Reply to myself : problem solved.
In fact, it was 2 problems. HDD was broken (confirmed in another computer) and keyboard flat cable was outside its port (so impossible to access BIOS pressing keys).
Solution to try : insert USB bootable flashdrive with Lubuntu (or another Linux distro) and WAIT A FEW MINUTES. I did'nt wait enough first times, I stop power source in less than one minute when I saw that nothing happened.
Computer will boot on USB and you will see if system works, proof of HDD failure.
Thanks to thojolu from HFR