I got it ! yeah !
Here is the way that worked for me, hoping it will be of some use for someone one day :
- Turn off computer
- Disassemble chassis in order to unplug battery and KRT battery (not sure if necessary)
- Push power button for 30s in order to unload any remaining electrostatic charge
- Put the power back
- Maintain F2 and press "start" for 30sec
Didn't worked the first time but after 2 trials, miracle, it finally worked.
Thank you everyone for your help.
Have a good day (I sure will).