Ocassionally things go bad(tm). This has bit me for the second time in approx 3 years now, but generally one needs to load the old FreeBSD kernel to start debugging and going through a box with a fine toothcomb.

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"
unload
load /boot/kernel.old/kernel
boot

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.