To understand your issue, you need to understand the Master Boot Record. It is "the information in the first sector of any hard disk or diskette that identifies how and where an operating system is located so that it can be boot (loaded) into the computer's main storage or random access memory."
So if you are not able to load into Windows, there's an issue with your MBR. Corrupted, Damaged, either way you'll need to reset it. There's various guides online to help you with that.
In the future, backing up your system with a disk imager (i.e. Drive Cloner, Macrium Reflect, etc) will make this easier, and if possible using a snapshot tool like RollBack Rx or Comodo Time Machine will prevent this from happening.