Ok, so we do a trick. (For Ubuntu and Debian)
I will use my old damaged Plesk Installation running on Ubuntu for DEMO (Proxmox QEMU):
1. Reboot Server
2. You should see GRUB Bootloader, Press "e"
3. You need to add on the end of the line what beginns with "linux /boot/vmlinux-(...)" this: "init /bin/bash (See Screenshot: reset 2)
4. After step 3. Press F10 or Ctrl + x to Start.
5. You should have shell access as root now, before we can reset password, we need to mount the filesystem.
6. Type: "mount -o remount,rw /"
7. Now we can reset Root Password: "passwd" and Press enter
8. Type your desired password, it will ask twice.
9. Continue booting to normal with: "exec /sbin/init"
10. You should be able to log in now
Tip: 5,6,7,8 you can see in Screenshot: reset 3