Windows 10 does some stupid things from time to time. If you have multiple drives in your computer when you install Windows 10 it can sometimes install the boot record on a different drive. If you remove this drive at a late time to upgrade it you will get a NTDLR press ctrl-alt-del to restart. Error and Windows will not boot. This is because your C drive has no boot record.
It’s an easy fix. Download Macrium Reflect Free and create a recovery disk using a USB stick. Follow the wizard it’s pretty straight forward. Now once you have your new recovery disk restart windows and boot from the new Recovery USB. Once it has restarted click on “Fix boot issues” and run through the Wizard again. Select your C drive and complete the wizard. This will add the boot record to your C drive and you should not be able to restart and boot into windows (remember to remove the USB after restart).