If you have the installation CD/DVD, reboot on the CD/DVD #1 and try a repair of the system (or of the boot loader only, I'm not sure if that choice is available) :
proceed as though you were to reinstall but choose the repair option when it shows this choice.
I hope that this will suffice to recover a running system, because otherwise it will be more difficult for you to tackle.
You will have to get a live CD/DVD of a distro like Knoppix
and reboot on it and try to reinstall GRUB on the MBR of your disk.
You can search for detailed instructions to do that, because this matter was treated many times before.
For future time you should think to backup your 35G of important data on a spare partition of your hard disk or on another hard disk.
Good luck, don't panic too much because it seems that you only have to reinstall GRUB, at least I hope so for you.