請參考FreeBSD Handbook,第9章第7節是這麼寫的....

The kernel does not boot:
If the new kernel does not boot, or fails to recognize devices, do not panic! Fortunately, FreeBSD has an
excellent mechanism for recovering from incompatible kernels. Simply choose the kernel to boot from at the
FreeBSD boot loader. This can be accessed when the system boot menu appears by selecting the “Escape to a
loader prompt” option. At the prompt, type boot kernel.old, or the name of any other kernel that will boot
properly.When reconfiguring a kernel, it is always a good idea to keep a kernel that is known to work on hand.
After booting with a good kernel, check over the configuration file and try to build it again. One helpful
resource is /var/log/messages which records the kernel messages from every successful boot. Also,
dmesg(8) will print the kernel messages from the current boot.

update kernel後沒辦法boot要做的第一件事應該是先用舊kernel讓系統開起來,而不是跑fix mode吧(因為有可能越修越糟)....

先找顆新硬碟安裝 OS, 再試著把舊硬碟 mount 進來看看可否還可讀取.