Please note: The main name servers for the ccTLD .is were not affected, since they are served by Anycast from all around the world in cooperation with Netnode in Sweden.
Any Changes to domains records, as well as new registrations, were not possible during the outage. ISNIC's monitoring system was also affected, which is why our staff was not aware of the problem until this morning. Every service had been restarted at 10:20 GMT this (Sunday) morning.
Work is ongoing to analyze the cause of the problem, and to implement changes to avoid same to happen again, and to minimize the effects of like or similar problems. A full "post-mortem" report will be disclosed before the end of this week.
Update 28.01.2022: a post-mortem is now available here.