
Use the menu rescue in the Distro installation disc or Rescatux
(Example Pending)

References:
Web: https://www.debian.org/releases/stable/i386/ch08s07.html.en
File: 8.7 Recovering a broken system.pdf
Topic: Reset root password with Grub2
Guide: Reset root password
Topic: Repair Grub
Guide: Boot Loaders and Dual Boot, section "- Repair Grub".