Make sure your computer is turned off, then turn it on. As it starts up, press the key to enter the Boot Menu (usually F12, Esc, or F2) and select the installation media.
Once your USB is ready, follow these steps to restore the missing file: windows 7 bootmgr download
While many people search for a "BOOTMGR download," you actually can't just download the file and drop it in. Since your computer won't start, you need to use repair tools to rebuild it. Here is how to get back to your desktop. 1. Check Your Hardware and BIOS Make sure your computer is turned off, then turn it on
to make a bootable CD/DVD that contains the BOOTMGR recovery tools. Use a Bootable USB : Tools like windows 7 bootmgr download