If I understand the problem correctly, I had a similar problem when there was a power cut, I would start the machine up and after the boot manager, the screen would go grey and fill with letters and beep like crazy.
I managed to fix it by using the FreeBSD installation disk, and copying over the boot loader from the disk into the boot directory, i.e. the boot files in /boot from the CD-ROM to my boot directory.
After that, I booted into single-user mode and ran "fsck". It seems to have returned to normal health.
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