In all of the 7+ years that I've had this Windows 7 Home Premium install on my rig, I've never had something so serious happen after a Windows update.
And there is more to it, because my Windows Boot Manager error was different:
Windows failed to start. Device is inaccessible.
I thought that it could have been my SSD suddenly dying or something, so I checked everything, the SSD is working just fine, but the repair procedure (which I could only enter by booting from the install media) initially said that it had found problems with the OS and detailed the fixes it was going to implement, but upon actually trying to repair, said that it couldn't write the repairs and then after that even claimed that there was no OS on the drive. I tried multiple things, it was a mess with multiple error messages, nothing worked until I found this post, which did the trick (thanks Brad!):
I had 3 Windows 7 UEFI boot 64bit computers affected by this patch yesterday. Here is the fix that I discovered that has worked for me. If the system boots to the automatic repair console, you can perform the fix from there, otherwise, you need to boot to an installation disc and run it from the recovery console. For my fix, I replace the winload.efi, winload.exe, winresume.efi and winresume.exe from a June copy that I found in a subfolder of the \windows\winsxs directory.
From the recovery console, determine the drive letter of your windows install (usually C) and change to it, it isn't necessarily the same as when windows is running - c:
dir winload.efi /a /s
It will find a bunch, but for me, the most recent one is actually at the end of the directory listing. Find the one with a date in June, mine was from June 12, 2019..
cd c:\windows\winsxs\C:\windows\winsxs\amd64_microsof t-windows-b..vironment-os-loader_31bf3856ad364e35_6.1.7601.24499_none_b98496 5a9ca0bb23
(No line break in the above command. You might find it useful to just type "notepad" in the command prompt so you can select the path from the CMD prompt and paste it into notepad, clean it up, remove the line break, add the "cd " to the front, and then paste it back into the command prompt.)
Once you are in the folder you found, if you type dir you will see the 4 files.
md c:\w (replace C with the drive letter of windows install
copy . c:\w
(should say 4 files copied)
ren win*.efi *.xxx
ren winload.exe *.x
ren winresume.exe *.x
You can then restart the computer and it should boot, finish the updates, and then allow you to log in. I tried checking for updates and it did not find any more so I don't fear that it will try to reapply the patch. I have seen no side effect to running with this version of the files. I replaced all 4 just to be consistent even though you might have been able to just replace 1 or 2.
Furthermore, other steps I tried that were determined unnecessary was to use DISKPART to assign a drive letter to the hidden system volume, and rename the BCD file in c:\efi\microsoft\boot and then rebuild the BCD with bootrec /rebuildbcd command. I did these steps on the first 2 because I had messed up the BCD trying to fix it, but on my 3rd computer I had not messed with and went straight to the steps I listed above and they worked on the first try. Hope this helps.
From what I could observe, it seems that the two problematic files are WINLOAD.EFI and WINRESUME.EFI, dated July 29. You should replace at least those two with the previous versions from June 12, following the instructions quoted above. And just to be extra cautious I only left the boot drive connected to the system during the procedure.
So, a miserable fail on Microsoft's part that cost me hours to fix because none of the usual repair methods worked (including command line bootrec /rebuildbcd, etc, etc).