When i got error message "BOOTMGR is missing", I tried many advices got online:
1. Reset BIOS default settings - done- error still there
2. Reset the hard drive - done - error still there
3. Run hard disk test- done-passed-error still there
4. Respond to 'hard disk not found' message - N/A, as I'm not getting it
5. System recovery - like I said, when I try to start system recovery I'm getting the error screen immediately.
...
At last, I find this tutorial, it helped me with its recommended tool "Windows Boot Genius" quickly!