So, I swapped a known working motherboard into the BSD machine. It now works. I also decided to use a quad core AMD AM2+ CPU that I had sitting around. That's all I changed.
I'd originally thought the problem was related to the hard disk. So, I decided the night before to disconnect the drives (it has two SATA drives), to determine if it were the real issue. It still experienced the same symptoms after boot-up attempts, which told me it wasn't a hard disk issue. I also swapped out the RAM with a known working chip with the same results when trying to boot-up.
So, either the the old CPU (a dual-core AMD...I forget the model) died or something on the motherboard died (or maybe there was a short somewhere?). I left it running a live instance of Linux Mint, just to see if it stays stable over the next 24 hours.
Next, I need to reinstall FreeBSD (wondering if I should try some others as well, such as OpenBSD or Mint). I wiped the drive, thinking that there was some corruption issue...shouldn't have done that.