Somehow your database got corrupted (power outages?).
You could try to repair it (/etc/pihole/pihole-ftl.db)
Or (might be easier) delete/remove it and start fresh over:
sudo service pihole-FTL stop
sudo mv /etc/pihole/pihole-FTL.db /etc/pihole/pihole-FTL_old.db
sudo service pihole-FTL start