Three ways to boot when the disk will not boot :
Use the Recovery Drive app to create a system recovery USB drive that can be used as the boot device.
The old methods of F8 or Shift+F8 still work, on condition that you are not using UEFI BIOS & SSD. In any case, if fast boot is configured then the opportunity window becomes too small to succeed (Microsoft link).
Use a boot CD/USB.
To be completely safe from all problems, best practice is to keep around a small recovery USB as well as a boot CD/USB (or be able to download one if disaster arrives), just in case.