One of the causes of the "BootMGR is missing Press CTRL+ALT+DEL to restart" is the need to cool and clean up your computer.
After 4 different times on occurring of the very same message and having applied all your suggestions with no more than a few hours or days success, I lost my patience and on the 5th time that it happened (the machine was working normally and suddenly
reboot itself and shows the damned message) and without engaging on any attempt to solve the issue, I simply opened the machine, carefully cleaned all the fans including the processor's with a gentle brush.
Miracle!!!
The machine simply started without any problems as if the issue never existed.
Conclusion:
Apart viruses, corrupt files or sections on the disk, or any other expert indicated problem, your machine can simply need a clean up and/or better cooling system for it (the damned message) also went away when I turned the AC on before implementing the
expert solutions for several times before success.