Look at this from Fedora FAQ -
Q:How do I reset my root password?
A: If you've forgotten your root password, and you want to change it, don't worry! It's possible. You need to boot into what's called "single-user mode." You must be _in front of the computer_ to do this -- you _can not_ do it remotely:
And here's that runlevel question -**
**While You Are Booting the Computer**:
- Using the instructions in the runlevel question (under the “While You Are Booting the Computer” section), boot into runlevel 1.
- Set the new root password with by typing: passwd
And then enter your new root password when asked.
3. Reboot your machine, and you will now be able to log in as root with the new password that you entered.
- When you first start your computer, the GRUB screen (where you choose your Operating System) appears. Select the Fedora that you want to boot into, but press the a key instead of pressing Enter.
- You will see a line somewhat like the following: kernel /vmlinuz-2.6.9-1.667 ro root=LABEL=/ acpi=on rhgb quiet
Add the number of your runlevel to the end of that line, and then press Enter. For example, to boot into text-only mode, the line would look like:
kernel /vmlinuz-2.6.9-1.667 ro root=LABEL=/ acpi=on rhgb quiet 3
You will then boot into the new runlevel this time only.
This just shows how easy it is hack into our systems unless we secure our grub!