BOOTMGR, the abbreviation of Windows Boot Manager, is a small piece of software that is loaded from the volume boot record. It executes winload.exe (Windows boot loader) to load important device drivers and core parts of Windows operating system. BOOTMGR helps your Windows 10, Windows 8, Windows 7, or Windows Vista operating system start. When BOOTMGR is missing, the operating system will not load, so you are unable to boot into Windows.