Boot up with antergos:rescue ISO
mount System to /mnt
You need to know which device is the right one check with
sudo fdisk -l
/dev/sdXn should be replaced with the right device information!
mount /dev/sdXn /mnt / root partition
mount /dev/sdXn /mnt/boot boot partition
mount /dev/sdXn /mnt/boot/efi the EFI-boot (fat) partition
BUT that grub is not in the boot list can be caused also by EFI-Boot-Settings not related to Grub and Antergos then, so re-/installing-/building grub may not solve your problem…
You can also make sure to get second (dual boot) installation rechecked and put into grub boot menu:
(as root also) If you have dual-boot os-prober may takes a long long time to work, the trick here is to mount all other partitions, before using it.
grub-mkconfig -o /boot/grub/grub.cfg
grub-install –target=x86_64-efi –efi-directory=/boot/efi –bootloader-id=Antergos-grub