This tutorial by FreeBSD user M.el Khamlichi shows us how to reset or recover your root password on FreeBSD 10.
Original post: http://www.unixmen.com/reset-or-recover-root-password-on-freebsd-10/

Let me show you How to Reset or Recover Root Password on FreeBSD 10.
My system:
root@Freebsd-unixmen:~ # uname -a FreeBSD Freebsd-unixmen 10.1-RELEASE-p6 FreeBSD 10.1-RELEASE-p6 #0: Tue Feb 24 19:00:21 UTC 2015 root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC amd64 root@Freebsd-unixmen:~ #Step 1: Start FreeBSD server/workstation.
Step 2: type number 2 key to access single user mode
Next you will see the following prompt from system:
When prompted Enter full path name of shell or RETURN for /bin/sh:
Press Enter key to boot into a single user mode. Next, you will be immediately dropped into a single user mode without a root password.
Step 3: Mount the file system
You need to remount / (root) file system in read and write mode with mount command, type the following commands:
# mount -u / # mount -aif you get this problem above . please fix it like this.
#fsck -y# mount -u / # mount -aStep 4: Setup a new password for root user:
# passwdNext type exit command to boot FreeBSD into multi-user mode environment:
# exit; exitOR You can just reboot the system:
# sync;sync # rebootEnjoy !!