At grub boot screen (after restart)
Select the kernel
Press the e key to edit the entry
Select second line (the line starting with the word kernel)
Press the e key to edit kernel entry so that you can append single user mode
Append the letter s (or word single) to the end of the (kernel) line
Press ENTER key
Now press the b key to boot the Linux kernel into single user mode
When prompted give root password and you be allowed to login into single user mode.