Do this at the log in screen:
Hold down the shift key on your keyboard while clicking the Power button on the screen.
Continue to hold down the shift key while clicking Restart.
Continue to hold down the shift key until the Advanced Recovery Options menu appears.
Click Troubleshoot
Click Advanced options
Click System Restore, see if you can go back to an earlier time.
If not, boot into the recovery environment again, then try a startup repair.
If that does not work, boot into the recovery environment, then click 'Go back to previous version of Windows'
If that does not work.
Go to a working computer, download, create a bootable copy, then perform a clean install.
Step 1: How to download official Windows 10 ISO files
[http://answers.microsoft.com/en-us/windows/wiki...](http://answers.microsoft.com/en-us/windows/wiki/windows_10-windows_install/how-to-download-official-windows-10-iso-files/35cde7ec-5b6f-481c-a02d-dadf465df326)
Step 2: How to: Perform a clean install of Windows 10
[http://answers.microsoft.com/en-us/insider/wiki...](http://answers.microsoft.com/en-us/insider/wiki/insider_wintp-insider_install/how-to-do-a-clean-install-of-windows-10/aef0ae63-2117-41ee-a8ea-4a3181625b08)
If there are files on the drive you want to recover, see - How to: Perform a Custom install of Windows 10
[http://answers.microsoft.com/en-us/insider/wiki...](http://answers.microsoft.com/en-us/insider/wiki/insider_wintp-insider_install/how-to-do-a-custom-installation-of-windows/38adfa8c-32f8-4354-8c53-13b5f2cf7e44)