The hard drive in my Windows machine that died and that I thought was the drive holding the OS actually wasn’t. It turns out, the surviving drive is the one with the OS, but the drive that failed had the boot record on it which is why I was unable to boot.
I was able to determine this by downloading the Windows 10 USB installer, booting up with that and inspecting the surviving drive. I managed to restore the boot information and boot successfully into Windows 10.
This is a relief. All that was on the other drive was game installs, most of which I can reinstall by downloading them from Steam or similar cloud libraries.