I don't see where you ever told us before that Windows will not start? When did this happen, after doing what exactly?
Follow whichever method will work here for How To Access Advanced Startup Options in Windows
8 or 10 . Note that Method 1 will sometimes work pressing Ctrl+Alt-Del to access Power icon in corner, to then press Shift + click Restart.
But without access to the OS you may need to
Create Windows 10 Installation Media on another PC to boot using the
BIOS Boot Menu Key, choose the boot device as a UEFI device if offered, then on second screen choose Repair Your Computer.
From the Troubleshoot options first try automated Startup Repair.
If it won't then start decide if you want to try to 1) fix what you have which at best is the inferior Upgrade install, 2) Go Back to Old OS, 3) rescue your files to do an automated Reset, or 4) do the vastly superior Clean install. For choices 2,3,4 skip
to the section after Safe Mode below.
To try to fix what you have choose Startup Settings, try to get into Safe Mode with Networking.
From Safe Mode w/Networking type Update in Start Search box to Check For and expedite Updates that may be critical but are taking their time to install, right click Start button to open Device Manager, import any missing
or erroring drivers from the PC or Device's Support Downloads webpage. How
to: Install and Update drivers in Windows 10 - Microsoft Community
If Safe Mode is stable the problem is most likely drivers, but could also need disinfection, System file repair or repair of the image. Most of the Upgrade problems seen are corruption passed from the old OS.
Download, install, decline trial version, update, then run full scans with
Malwarebytes and
AdwCleaner.
Uninstall any bloatware anti-virus like Norton, McAfee, AVG or Avast, enable built-in Defender to test performance. Use the special
Uninstallers (removal tools) for common antivirus software - ESET Knowledgebase.
Right click the Task Bar to open Task Manager, on Startup tab disable everything until your install stabilizes, then only restore those you know for sure you absolutely need starting with Windows – none of them really need to unless you want them.
From the Advanced Options Command Prompt, or within Safe Mode accessing an
Elevated Command Prompt,
Copy and Paste or type each of these commands, each followed by Enter:
sfc /scannow
Dism /Online /Cleanup-Image /RestoreHealth
If it prompts for Sources file, mount or insert Windows 10 installation media, locate install.wim or .esd file in Sources folder, substitute it’s path in the following command (for example: Dism /Online /Cleanup-Image /RestoreHealth /Source:wim:H:\Sources\Install.wim:1):
Dism /Online /Cleanup-Image /RestoreHealth /Source:wim:Full Path to install.wim file:1
(or) Dism /Online /Cleanup-Image /RestoreHealth /Source:esd:Full Path to install.esd file:1
sfc/ scannow
Exit
If at any time you are returned enough functionality but still have problems with an app then
Troubleshoot problems with an app - Windows Help .
If Safe Mode doesn’t help stabilize things then return to
Advanced Startup Options in Windows 8 or 10. Next try System Restore to a stable state,
Command Prompt to run above commands including sfc /scannow,
Go Back to Old OS, or Reset Windows 10.
Better yet do the vastly superior
Clean Install Windows 10
which will reactivate if the Upgrade has already activated, or in Fall Update Version 1511 can directly activate with the Windows 7, 8 or 8.1 Product Key (even if embedded) after install at Settings>Update and Security>Activation>Change Product Key.
Activation in Windows 10 - Windows Help
At any time you can rescue your files, which should always be backed up externally if you value them, by booting into a Windows installation disk to
Copy & Paste - in Windows Recovery Console, a Linux disk, or
Paragon Rescue Kit Free Edition File Recovery Wizard to copy files to USB flash
stick or an external or another hard drive.
If these fail and/or you decide to go back to your previous install you can try to run Factory Recovery from it’s hotkey at boot; find yours by googling the “PC make/model + Run Factory Recovery.” If none then do a
Clean Reinstall Windows 7 or
Clean Install - Windows 8 deleting all partitions during the booted install using the Drive
Options on the “Where do you want to Install Windows?” screen.
Feel free to ask back any questions and keep us posted on how it goes.