How to fix BOOTMGR is missing error without a Windows disk
You will still need a boot CD/DVD or USB drive, but not with the operating system distribution. You should place some recovery image on that drive, Hiren’s Boot CD for example. The images of these disks are available on most torrents and include a set of tools that allow correcting some windows issues including bootmgr is missing error.
Here is the list of software you can fix “BOOTMGR is missing” error with
Acronis Disk Director
Acronis Recovery Expert