(1) Make Win 10 Installation Media.
https://www.microsoft.com/en-us/software-download/windows10
Download Windows 10 - Currently, this is v.1803
Make a Windows 10 Installation USB drive or burn an ISO to DVD. As the site says,
match it to your Edition (Home, Pro, etc.), Language, & Architecture (x64/64-bit or x86/32-bit) of the computer on which it will be used. This may be used to do a clean install or a repair-install. Also, it comes with its own recovery environment (Choose an Option screen).
(1)
Backed up your files & downloads to external media.
(a) Plug in the Win 10 Installation Media. Also plug in a USB drive on which to copy files.
(b) Boot to the Installation Media through the
BIOS Boot Menu. You press a key during boot at the manufacturer's logo screen. Find your key here...
https://www.sysnative.com/forums/hardware-tutorials/23010-get-access-bios-settings-boot-menu-f-keys-hotkeys-system-manufacturer-oem.html
Get access to BIOS settings & Boot Menu: F-keys/Hotkeys by System Manufacturer (OEM)
Note: That's a 3rd-party site. The literature is good. I know nothing of any downloads or links at that site.
(c) When the Installation media boots, get to the 2nd screen, & click "Repair your computer". This will boot to the Recovery Environment (of the Installation media) at the "Choose an option" screen.
(d) Click "Troubleshoot > Advanced Options > Command Prompt".
A DOS box will open.
(e) Enter "Notepad" into the DOS box. It will open in full GUI mode.
(f) Select "Open" from Notepad's File menu.
(g) At the bottom right of the Open requestor, set it to read
All Files...

(h) Now all files should show up. Don't try to open any of them except maybe text files. But browse to - especially - "C:\Users<Username>", right-click it in the left pane, & select "Copy".
Then click the letter of the USB drive in the left pane (not the one that holds the Installation media). Right-click in the right pane, & select "Paste". Do the same for "C:\Windows.old\Users<Username>", if it exists. That
would get all the stuff of the user. I guess you could be more selective, & just take the subfolders that hold your data.
(I) Close the "Open" requestor, Notepad, & the DOS box.
This gets back to the Choose an Option screen. Unplug the USB external storage drive.
(j) I guess try "Troubleshoot > Reset This PC".
Be sure to keep all files & apps, if it mercifully asks the question.
If that fails to fix things, then a formal clean install is in the offing.