You can log in with single-user mode, in which
you can create a new root password.
When you see the boot: prompt, type linux
single to enter single-user mode. Some of the filesystems
will be mounted, and you will find a bash# prompt
when you've entered single-user mode (note that this prompt will look
somewhat different than the prompt you're accustomed to).
Now, you can change root's password by typing
You'll be asked to re-type the password for verification. Once you're
finished, the password will be changed and you can reboot by typing
shutdown -r now at the prompt; then you can log in
to root as before.