HiveSQL is a service that collects all the infos from the blockchain to make it easily to search and create overviews.
This is a time saver for all the apps who use it. It's expensive to run and afaik it runs very solid.
I don't know what happend with that downtime but think of a replay of all this blockchain data into a structured database - it might take some time.
I heard that the HiveSQL was hacked :(