I had the exact problem. My computer was working perfectly fine last night, and when I woke up, it had a black screen that said missing the bootmgr. I did a lil search online and followed a couple of instructions. Non of them worked for me.... I finally found out that the computer wasn't recognizing my Hard drive. I have two hard drives, One for boot windows OS, and the other for extra space (pic, vid, mov, games)...
So I guess that this goes for the people who have extra hard drives and have the message that says: bootmgr is missing...
Go into bios and make sure your booting from your CD: first and then from your Hard drive. Then make sure that your bios recognizes your main hard drive. If it does not, then disconnect everything from your computer. Open the back and make sure that all the wires are properly connected to your hard drive. If you have SATA cables connected to your hard drive, make sure that they are not bent. The bent causes the motherboard not to send and receive information from the hard drive. I would recommend to unplug both, the power and sata cable from your hard drive and just reconnect them. That is what i did and I restarted my computer and WHALLA, it worked... Thanks for reading!