If it will not boot at all ... even to safe mode, you need to make a Windows 10 install boot disk (on DVD or USB Drive) and boot to it. When you get to the part requesting whether or not you wish to "Repair Startup", go there. There are several things you can do there including System Repair. You may even be able to repair it without System Repair. For more information on this, google "startup repair". Good luck. I've been there and survived.