Start system, wait for grub-menu. Use cursor-keys, move highlight to os-booting-menuentry. Press key 'e'. Look for line beginning with 'linux'. It will look something like this
linux /boot/vmlinuz-3.16.0-4-amd64 root=UUID=a6cadc36-b8fa-4a35-896a-a6b882dab26e ro quiet splash
Edit this line, change 'ro' to 'rw'. Add 'init=/bin/bash' after 'rw'. Delete all other boot-options. So the line looks like this
linux /boot/vmlinuz-3.16.0-4-amd64 root=UUID=a6cadc36-b8fa-4a35-896a-a6b882dab26e rw init=/bin/bash
Press key 'F10', system will boot to '#' promt, (the DE will not load). Enter command, (change 'user-name' to your user-name)
# passwd user-name
Enter new-password. Re-enter new-password. You should see reply : 'success'. Enter command
# sync
Reboot system, (press reset-button). Login with new-password.
linux /boot/vmlinuz-3.16.0-4-amd64 root=UUID=a6cadc36-b8fa-4a35-896a-a6b882dab26e ro quiet splash
Edit this line, change 'ro' to 'rw'. Add 'init=/bin/bash' after 'rw'. Delete all other boot-options. So the line looks like this
linux /boot/vmlinuz-3.16.0-4-amd64 root=UUID=a6cadc36-b8fa-4a35-896a-a6b882dab26e rw init=/bin/bash
Press key 'F10', system will boot to '#' promt, (the DE will not load). Enter command, (change 'user-name' to your user-name)
# passwd user-name
Enter new-password. Re-enter new-password. You should see reply : 'success'. Enter command
# sync
Reboot system, (press reset-button). Login with new-password.