If ever we find the terrible error "BOOTMGR is missing" we must remain calm. The solution is simple indeed.
This is an error that appears when you start the computer and thus leaves start Windows. This happens because it has been damaged Windows startup somehow.
To do this, we will repair the BCD. The BCD (Boot Configuration Data) is the Windows boot configuration. Here's how.
How to repair the BCD
-Iniciamos Recovery mode in the system. For this we turn to a CD made from another computer with Windows, have a full Windows CD or a special key on your computer (let's try F8)
-Once We agree to this, we seek the Command Prompt or command line.
-In Her, we write: bootrec / rebuildbcd
Theoretically we should announce that it has repaired properly. You can also ask to add our system BCD, then pointed Yes / Yes.
As you see, it is a simple process but it is delicate. The hardest part may be familiar with these tools.
If you do not get so repair is likely to be another error or finally have to reinstall Windows to complete.
If you are in doubt about this process not hesitate to ask since you can finally start destroying more than it is.