If there's any good news to these frequent reports of machines that are unable to boot after applying the patches, it's that the problem seems to mainly affect older AMD processors, like the Athlon X2 6000+. Given the many reports of [now] inoperable systems, Microsoft has taken the security patches for Meltdown and Spectre offline.
Microsoft provided the following statement with regards to remove the patches: "Microsoft is working with AMD to resolve this issue and resume Windows OS security updates to the affected AMD devices via Windows Update and WSUS as soon as possible. If you have experienced an unbootable state or for more information see KB4073707."
Interestingly, following the link to the Knowledge Base article gives a bit more information on the issue, and even blames AMD for putting users in this predicament:
After investigating, Microsoft has determined that some AMD chipsets do not conform to the documentation previously provided to Microsoft to develop the Windows operating system mitigations to protect against the chipset vulnerabilities known as Spectre and Meltdown.
If you have an AMD-based machine and haven't downloaded the Meltdown/Spectre security patch, you won't be able to install it until Microsoft and AMD can come together to deliver a workable solution. Yes, that will leave your system vulnerable, but the upside is that no known attack vectors are operational to take advantage of Spectre and Meltdown... yet.
For customers that now have a gimped system after applying the update, Microsoft is providing the following resources to help you get back on track.