Thankfully I've solved this through reading around, the forums - May work for you.
I'm Win 10, Sony Viao (so getting on a bit) - I was about to hit the reset!!
I was not satisfied with completely giving up and basically starting from scratch and loosing all my configured setup.
Quite Frankly I'm fairly disappointed with the general response on this issue, from Microsoft.
So in summary I was able to get to a black desktop, with a re-cycle bin and nothing else useful or responsive including the Windows/Start Button/Menu
From this ' broken' desktop CTRL-ALT-DEL gave me access to Task Manager.
From Task manager I found out I can run some programs (File >> Run New Task)
From this I was able to navigate and run new a program. Including CMD
Bit like this below.
Now the general guidance to try these two commands DISM and sfc seemed hopeful but getting there was hard work, BUT run CMD above and open a command window (i guess with admin rights, why not) you can run these commands by just typing them in.
DISM.exe /Online /Cleanup-image /Restorehealth
and then
sfc /scannow
Obviously I went through these
first DISM seemed to report that it had fixed all (I think that is its default response for changing nothing!)
sfc reported no problems.
and lo and behold I got to the
C:\WINDOWS\system32\config\systemprofile\Desktop is not accessible Access is denied
So basically the same state (now i'd been round this loop before!!) from :
sihost.exe - System Warning - Unknown Hard Error >> to >> Desktop is not accessible Access is denied >> couple of re-boots
So my breakthrough was ...
I managed to find here
https://support.microsoft.com/en-us/help/12415/windows-10-recovery-options
which I followed to
Use installation media to restore or reset your PChttps://www.microsoft.com/en-gb/software-download/windows10
and
and following
Follow these steps to create installation media (USB flash drive or DVD) you can use to install a new copy of Windows 10, perform a clean installation, or reinstall Windows 10.
I was able to 'Create Windows 10 installation media ' install on an 8GB USB stick.
NOW ... my nugget that finally worked for me ...
All the guidance appears to be to run from safe boot*, but this would not allow this particular USB re-install to work, because it needed to run from a desktop.
Well I had a broken desktop!**
So running task manager I navigated to the USB drive and run Setup at the root of the USB drive and followed it through to reinstall Windows 10..
Took some time but everything is back - Happy days
*safe boot (hold down shift, whilst selecting the windows power button at a logon screen and selecting rest)
** now I was a little tight (12GB) on hard disk space (12GB, tight!! when I was a kid, on by 286 with 20MB Elonex!! ... any way)
so I made sure I had enough by deleting stuff that I could do without through again using the task manager browser and looking for large files and directories in my user docs that were definitely not needed, properties worked to tell me the size. I did not need to try copying to another USB stick to backup, but that may be an option)