I'm Greg, an installation specialist and 8 year Windows MVP, here to help you.
- Try whichever method works here to get into Advanced Startup Options to Go Back to Old Version: https://www.tenforums.com/tutorials/2294-advanc...
If you are able to access Go Back and it fails, skip to step 3.
- If necessary create bootable installation media to access the repair options if not accessible by any other method: http://windows.microsoft.com/en-us/windows-10/m...
Boot media by powering up the PC while pressing the BIOS Boot Menu Key: https://www.sysnative.com/forums/hardware-tutor...
You may need to enter BIOS/UEFI Setup to turn off Fast Boot if it won't allow the Boot Menu key to interrupt starting Windows: https://www.mydigitallife.net/comprehensive-lis...
Choose UEFI version of media if offered, browse to second screen to Repair Your Computer link, then Advanced Troubleshoot options - to Go Back.
- If this fails try a Startup Repair, or go back to Troubleshoot Options on the previous screen to Reset your PC which will save your files while reinstalling Windows.
- If that fails you can use the booted media to do the Clean Install. This link compiles the best possible install of Windows 10 that will stay that way as long as you stick with the tools and methods given. http://answers.microsoft.com/en-us/windows/wiki...
There is a step to rescue files from the booted media if they are not scrambled: http://answers.microsoft.com/en-us/windows/wiki....
Then be sure to wipe all partitions off the drive to get it cleanest.
- You can also run the PC's Factory Recovery from it's hotkey at boot, following the procedure for that model found in it's Manual on it's Support Downloads webpage, or by asking Google how to Factory Recover the exact make and model.
Feel free to ask back any questions and let us know how it goes. I'll keep working with you until it's resolved.