So, to make sure.
Since my previous installation have damaged the generic Mbr that i had (am I right?), I should use boot repair to restore it on /sda.
When looking at the diagnostic that I posted, I saw no boot loader in the MBR of sda. Is that what caused the booting issues.
Just wainting for confirmation now.
Processus: Using boot-repair:
Select restore Mbr. At this point in the Mbr options must I keep in the "partition booted by the mbr" choice, sda2 which is my / ("linux core partition")?
Lauch the repair. Boot-repair will restore my mbr, uninstall grub from its current location and then reinstall it into the Mbr.
Finally, my computer should boot on my Mbr which will boot the ubuntu partition.
Sorry if it takes time but I 'm quite strarting from scratch here when it comes to Ubuntu.
Also thanks for helping!