Find two flash drives that you can format (> or = 8 GB).
On a working computer create a bootable Ubuntu flash drive and a bootable Media Creation Tool (MCT) or Windows 10 iso.
https://ubuntu.com/tutorials/create-a-usb-stick-on-windows#1-overview
https://www.tenforums.com/tutorials/2376-create-bootable-usb-flash-drive-install-windows-10-a.html
Boot the problematic computer to the ubuntu flash drive:
Use the Ubuntu Graphical User Interface (GUI) to copy and paste all important files to another drive or the cloud.
After backing up all important files you can decide whether to spend time troubleshooting or whether to perform a clean install.
https://www.tenforums.com/tutorials/1950-clean-install-windows-10-a.html
https://www.tenforums.com/tutorials/9230-download-windows-10-iso-file.html
https://www.microsoft.com/en-us/software-download/windows10
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