How to reset root password in single user mode in Debian ?
At grub-boot loader screen. Select Recovery mode the version of the kernel that you wish to boot and type e for edit. Select the line that starts with kernel and type e to edit the line.
Go to the end of the line and type init=/bin/bash as a separate one word (press the spacebar and then type init=/bin/bash). Press enter key to exit edit mode.
Back at the GRUB screen, type ‘ b‘ to boot into single user mode
This causes the system to boot the kernel and run /bin/sh instead of its standard init.
Now you have gained root privileges and a root shell. Since / is currently mounted read-only and many disk partitions have not been mounted yet, you must do the following to have a reasonably functioning system.
# mount -n -o remount,rw /
# mount -avt nonfs,noproc,nosmbfs
# cd /etc
Now enter the new password and reboot the server and then you can able to access your server with new password.