If you have one pc and it seem bootmgr missing then follow below step to avoid format pc.
You must have another one pc for stating this activity,
First shutdown both pc.
Then remove Hard drive (HDD) from computer that you have facing “bootmgr missing”.
After remove HDD it connect on working PC as secondary,
Then power on the computer and start the operation system,
After open windows Go to Computer Management–>System tools–>Disk Management,
You see there are two hard drive connected on pc Disk 0 and Disk 1,
Disk 0, is local pc Hard drive and Disk 1 is another pc drive ,
Click on Disk 0,You see the so many drive in it,
if your pc have two drive than it display three drive another one extra drive is System reserve drive so right click on it,
After right click on “Change Drive and Letter and Paths”
Click Add button and then select anyone letter (E.g P) that you have to display as drive and then press ok.
Same active doing on Disk 0
After Open two system drive ,just open my computer there are so many drive are display so if you have add P drive as local pc drive and Q drive is “bootmgr missing” pc drive
So open P drive and you see the “bootmgr” file just copy it and paste in Q drive
After paste the file Shutdown pc and disconnect another pc hard drive and reconnect the pc that have to issue
After connect just power on it is working.