This will fix on a standard installation, if you do manual partition, ask at the forum!
Boot up with our LiveIso.
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, and depending on your installation you may have or have not a seperate /boot partition. If not /boot is a folder under / on the same partition as the filesystem root.
mount /dev/sdXn /mnt >> / root partition
mount /dev/sdXn /mnt/boot >>> /boot partition --> without extra /boot partition you do not need this step!
To make shure to get second (dual boot) installation rechecked and put into grub boot menu use os-prober, (for Antergos only system you go directly to grub-mkconfig)
(as root also) If you have dualboot 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=i386-pc /dev/sdx
Where /dev/sdXn should be changed to what you get into your system: check with:
$ sudo fdisk -l
For EFI Boot systems go here: https://antergos.com/wiki/miscellaneous/how-to-fix-grub-with-efi-boot/