You can boot the live system and then 'chroot' into the installed system to
do whatever needs to be done to fix the problem (I assume you need to edit
/etc/default/grub and run 'update-grub'). That is not that easy for a
GNU/Linux beginner.
If you feel brave enough, read this documentation (and come back here for
some directions):
https://help.ubuntu.com/community/Grub2/Installing#via_ChRoot (forget about
items 3-5 and 8-9; replace item 12 with your fix)
Otherwise, search for someone to help you (a friend?, a LUG nearby?, etc.).
