Gacrux

good luck

the partition doesnt exist? what does lsblk say?

if the partition really doesnt exist then you either deleted the partition or destroyed the partition table. either way, if you dont have important data on it then id recommend you just abandon the install, delete everything and make a new one. otherwise you could try your hand at some linux file recovery tools (windows ones are almost certainly a scam).

been a while since i done this so i might have gotten some parts wrong:

1. get a thumbdrive/usb drive/whatever you call it
2. install the arch iso on it. im assuming you still have another device you can boot to.
3. plug in the usb into your broken device. turn it on. then boot into that usb. you probably did this while installing arch so im assuming you know how to do that.
4. once in, mount your broken arch install by doing mount /dev/<whichever partition it was> /mnt. take note of the space between the partition and the /mnt.
5. then do arch-chroot /mnt
6. from there just install grub like normal (just do whatever you did to get grub working in the first place).