The following snipbit gives you an idea what steps to take when rebooting the server so that you can load the previous working copy of the FreeBSD kernel:
When the boot menu appears hit the spacebar to stop the countdown.
Press "6" for "to escape to loader prompt"
Now the old working FreeBSD kerenl is booting up. It would be recommended to copy the last working version to /boot/kernel.last for example so that you can "load /boot/kernel.last/kernel", especially if you are going to be building your kernel multiple times on a server.