"BOOTMGR is missing"
This is an easy fix, no need to panic, more then likely you just need to repair your windows or re-install it which ever your choosing.
You should have a copy of your Windows disk that came with it. If not I am pretty sure someone you know has one you can borrow. As for the CD KEY, look under your laptop or on the back or side wall of your computer desktop.
Image credit to: www.technibble.com/
The product key is what is used to activate your Operating System.
I know with laptops over time the information will rub out. its always good to keep a record of it.
Contact me if you need some help solving this issue.