Remove database lock:
When the package manager is about to alter the package database, for example installing a package, it creates a lock file at /var/lib/pacman/db.lck . This prevents another instance of a package manager from trying to alter the package database at the same time.
If the update got interrupted while changing the database, this stale lock file can remain. If you are certain that no instances of a package manager are running then delete the lock file:
sudo rm /var/lib/pacman/db.lck && sudo pacman-mirrors -c all && sudo pacman -Syyu