Hi Marco..

I realize this is your first time posting. When you posted, just above the subject, in red letters it read:

Note: If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem.

Perhaps, you might share if the message "suddenly" appeared, or did something lead up to it? Had you tried to install something? Were you having any problems prior to this? Did you try System Restore at any point. I realize you said you're unable to boot, but I'm trying to get an idea of things you might have tried before this happened.

We can tell you how to do a repair install, but it might help at this point to get a bit more information. In this case "more is better".

Carol