When you fail to boot into your Windows computer, and there is an error message on the display like “boot manager is missing” or something similar, you need to fix your boot record to make it works again.
Somehow, the boot record on your hard drive is missing, and Windows is unable to boot it, or at least locate the boot device. You can also use Windows Repair feature that comes with the installation CD/DVD or bootable USB flash to repair your Windows computer and get it back to work like before.
Sometimes, the configuration partition on your hard drive is corrupted or deactivated. In this situation, you need to use a tool like Partition Wizard to reactivated this partition. You also can re-enable this partition with a command.
After resolving this issue, you should do backup more frequently to make sure you will not lose any file when something happens to your Windows computer.
I would suggest you to setup RAID to mirroring your hard drive if you have two hard disks at the moment. It will help you protect your data more safety without losing any file. The RAID 0 will copy the same file to both hard disks. If something happens to one hard drive, the other still works.