Unless it is a matter of you entering bios and making sure grub is loaded before windows the following is probably required.
You most likely have to boot with a live iso, like the one you installed with.
Then you need to chroot and run update-grub. That would most likely find the manjaro boot partition and fix grub, unless you did something strange duing the win10 installation that removed the partition, then you need to do more stuff.
You can find a bit more info by reading this. (only do the chroot part and then update-grub, you should not have to reinstall any kernels)