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/bash instead of its standard init. This will allow us gain root privileges (w/o password) and a root shell.
If you wanna reset root password remount paritions
mount -rw -o remount /
Setup root password