So I reseated my RAM and daughter cards and unplugged my data HDDs and removed a stick of RAM and attempted to boot but got a 'Disk error. Press any key to restart' message. I plugged all the HDDs back in and got the same thing. I switched the SATA cable *back* to the old one and went into the BIOS to check the disk priority and noticed that some allusion to a USB drive was in first place. It may have been like that for a long time or may have just occurred. In any case, I put the SSD in first place and saved to CMOS and the computer started up just fine. I don't really know what happened. Windows reported not shutting down properly but I don't know why that would have been. The problem is resolved but I'm not any wiser. :/
Edit: When putting the case side panel back, I unplugged the old SATA cable (which is the reason I replaced it with one which has clips to prevent accidental unplugging) so I suffered the 'Disk error' again. After switching back to the new cable, I still got the error, so I ventured into the BIOS and saw that disk priority had changed again. I raised the SSD to the top and now it works again. Nonetheless, the cause of the original error is still a mystery.