Do not Panic! Simply restart the system, type boot -s at the Boot: prompt (just -s for FreeBSD releases before 3.2) to enter Single User mode. At the question about the shell to use, hit ENTER. You will be dropped to a # prompt. Enter mount -u / to remount your root filesystem read/write, then run mount -a to remount all the filesystems. Run passwd root to change the root password then run exit(1) to continue booting.