Hi pentimentosound
As was mentioned .. first port of call try a LINUX LIVE Distro DVD/CD. You can make one of these free if you have a CD / DVD handy and another puter to do it on. Try UNBUNTU Live here, download the ISO and burn:
https://www.ubuntu.com/download/desktop Once you have burned that ISO, boot your PC off that CD/DVD .. It will run LINUX op Sys from the CD/DVD. It will reach a point asking you do you wish to install UBUNTU to the Hardrive OR Run it off the CD/DVD. Pick the latter :D
Then you will end up with the Linux desktop running off the CD/DVD and not interfering with your Windows Drive or Data.
The Linux D/Top is pretty self explanitory .. Go for the File manager over left and you should be able to see all the folders and files on your C Drive. You can then, if you wish, Backup to another USB Drive you just attached, Linux will see that immediately.
The other Q of course is .. Could this be MALWARE / VIRUS? There are many Free Anti Virus Distros running off a live CD/DVD also. Here is a recent article on these:
https://www.lifewire.com/free-bootable-antivirus-tools-2625785 So again, download, burn, boot off one of these and run an AV test on your drive.
Finally, i assume you tried booting off a Windows 10 install CD/DVD? To get/make one:
https://www.microsoft.com/en-gb/software-download/windows10 Go for the DOWNLOAD TOOL NOW and run that on another PC. When run it will ask do you want to upgrade in place this PC OR Create an ISO / Win 10 Media file to burn to a DVD. Again take latter option. Then boot your broken puter off that and at some point it gives an option to REPAIR WINDOWS, down the end in smaller writing, keep an eye :D
Hope that lot helps .. Aidan