So did you only have 6.3 installed?
If not have you tried booting other ones in case they were not interrupted?
If you have a live usb handy then you should be able to continue from there.
Though if you have encryption, yes, that will require extra steps.
See for example here:
Guide here:
Once you can successfully chroot you may want to investigate /var/log/pacman.log.
You may wish to wipe out all of the cache
paccache -rvk0
Or just the offending package:
paccache -rvk0 linux63 linux63-headers
Then continue the update and specifically target the previous problem package, ex:
sudo pacman -Syu linux61 linux61-headers
If you use mkinitcpio then rebuild the initramfs with that:
sudo mkinitcpio -P
And update grub:
sudo update-grub
You should be ok to exit chroot and restart:
exit