What is the bootMGR is missing under win7 system?

Your problem is because the C drive is set to the active partition under win7. After the C drive is set to the active partition, the space of 100MB under the original win7 becomes an inactive partition. When the system starts, it cannot be found under the C drive. Boot file.

Because the system has been unable to start, after this failure, you need to use WinPE to boot,
After entering the PE, in the disk management, the right system retains the partition, select: mark the partition as the active partition, Then restart to enter the system.