The complete steps to reset root password for sandbox is
1. start your VM
2. press f12 to go to grub menu
3. press e
4. select the second line "kernel /vmlinuz-2.632 ...."
5. press again e
6. add s at end of line (<consoleblank=0 quiet s)and press enter.
7. press b which will log you in as root without password.
8. Change password using passwd. Be carrefull, you are in qwerty mode
9. press exit and press entrer