Before we proceed (and this should be the first thing we need to know),
what is your disk partitioning (gpt or msdos)
what is your windows booted in (uefi or bios-legacy)
I assumed your disk is msdos and your windows bios-legacy too.
Can we confirm this?
Since you cannot boot manjaro, can you find out in windows?
At this very point, my windows know-how is suddenly zapped out.
can you find out or search for us and come back to us?
We certainly do not want you to do something however strong our assumption is.
ps: probably you have to reinstall. sorry.
and there's lots like yours (boot partition in vfat)
It installs and runs (for some time) but ultimately it will fail.
 - you must install in same mode as your windows (unless you want to zap windows)
if windows is uefi, make you manjaro uefi
if your windows is bios-legacy, make manjaro bios-legacy.
if gpt, use uefi
if msdos, use bios-legacy