I also had this problem several times in the last weeks. I did a lot of new installations in different partitions, mostly EOS-Kde with systemd-boot. after following updates the black screen appears instead of the boot-menue. Only live boot ISO from a stick works after that. But I was not able to repair the boot mechanism with arch-chroot etc, everything looked correct for me. Following method always helped me:
boot live ISO.
resize a partition to make little space
create a small new partition e.g. 20GB
new installation into the new partition
reboot after that as with any new installation
The boot-menue appears again and all entrys can be selected as before, be happy.
I have the impression, that the black screen is because the boot is trying to start from a wrong drive. The new installation repairs that somehow.
Next time the new installation into the same small partition is more easy.