First, hello, and welcome! Second, I'm sorry you had this problem. Third, I'm not a FreeBSD user, so my advice may either have already been tried, or ... be entirely useless to you.
- Boot a FreeBSD system capable of running bsdlabel(8). This could be your installation media.
- From the root shell, use bsdlabel(8) with the -B option, to install new bootblocks from /boot/boot.