I have encountered the "bootmgr is missing" error with my Windows 7 computer and I was not able to boot to Windows beforehand either. I am currently attempting to recover my files before I reformat the Windows OS on my C: drive. That said, I am booting from a USB using Ubuntu and am in the "trial" version (just trying it without installing it).
I do not see my hard drive or the drive that would presumably be my C: drive in Windows when I look through the devices. I am not sure where my files may be but I am fairly certain they have not been deleted or formatted over.
I am strictly looking for ways to recover the files to an external drive and not so much how to dual boot or permanently install Ubuntu.