Thank you very much both for answering!
This morning when I tried to boot i went directly to the GRUB-terminal. Using this
https://www.linux.com/learn/how-rescue- ... ub-2-Linux , I got Grub started, but as soon as I started Linux it said that it had to reboot because a new graphic-driver had been installed and then I have only gotten a black screen.
I am only a graphic artist so I have a hard time figuring if this is a GRUB/BIOS problem, something related to my two-screen setup (a wacom-tablet) or some kind of hardware failure on my harddisk. Everything worked and I am not sure if I in all my failed attempts has upset something or it is a material failure.
I appreciate your advice and will try to come up with some answers if I get the chance, but I think I have to buy a new harddisk and try to set up a new system regressing to Mageia 5.1 and then move files from my old harddisk or if that is not possible from my backup.