When I updated my Linux kernel to 5.11 I had the bad surprise to end up with a blinking underscore on reboot. It had been many years since an update had broken my system like that. I fixed it rather easily by booting in rescue mode and downgrading the kernel. I had no time to investigate so I just added
IgnorePkg at the time, But I don't use Arch to run old kernels so today I took the time to fix it "properly".
To do so, I reproduced the issue, then downgraded again and looked at the logs with
journalctl -b --boot=-1. It quickly let me understand that it was GDM that was failing due to something wrong with graphics initialization.
mkinitcpio.confand rebuilding the initramfs, that fixed it for me.