Thanks for your encouragement! I checked the menu.lst, and then I also try with menu.lst.old, which I renamed as menu.lst. Finally I read https://help.ubuntu.com/community/Grub2
in which says that grub2 doesn't use menu.lst. I also found what the problem was: At running upgrade-from-grub-legacy, we have to select one of the choices, ... I didn't know how: I tried with TAB, I tried with an asterisks, and finally I tried with ENTER. By pressing "enter" none of the choices were selected and I ended with Error 15, no file found. If I had only knew that I needed to press "space bar" to select one of the options...
I mounted the system on DSL and tried to run again /mnt/hda1/usr/bin/update-grub , and I got:
/mnt/hda1/usr/sbin/grub-mkconfig: line 94: /usr/lib/grub/grub-mkconfig_lib: no such file or directory
Then I tried the chroot command (that was something new for me)
FATAL: kernerl too old
Then I tried again with DSL-N which has kernel 2.6 (I didn't know that), but I got the same result.
Finally I tried with "system rescue cd" (Also new for me), this has many options to boot, and I selected to boot with one of the installed operating systems. And there you go!, I could get Debian again. I had a look again at http://www.howtoforge.com/upgrade-de...w-simple-steps
and tried to run some of the commands at the end of the page and finally ran upgrade-from-grub-legacy and selected both options.
Now Debian started again... in console mode with the error:
Failed to start the X server (your graphical interface). It is likely that it is not set up correctly. Would you like to view the X server output to diagnose the problem?
Again... I don't know where to go from here.
But thanks in advance, interesting adventure so far...