A new update KB4073578 is now available for Windows 7 users. This latest update will fix unbootable state for AMD devices.
Recently, Microsoft rolled out a monthly roll-up update KB4056894 and a security-only update KB4056897 to fix the chip-level security flaw(Meltdown and Spectre) in Intel processors. However, it created problems like unbootable state for devices running AMD processors.
Windows 7 update KB4073578 download Links
Windows 7 security update KB4056897 is also now available here.