The decision was made this morning with mystery surrounding how the virus has spread, with the current assumption it is a result of community seeding.
'We are telling the community to stay at home,' CCA chairman Peter Ferbrache said.
Essential shopping and two hours of outdoor exercise is allowed.
That should take place with household members unless you live alone, in which case you can exercise with one member of another household while socially distanced.
Covid vaccination appointments will continue.
Measures will be in place for 'some time', but reviewed after two weeks, Deputy Ferbrache said.
Schools, colleges and pre-schools, nurseries and childminders will close except for vulnerable students and children of essential workers.
People should work from home if they can.
Restaurants and non-essential retail are not being treated as essential and should not open, even for deliveries or takeaways.
'The safety of our community has always been our priority. The reappearance of cases of Covid-19 via unexplained community transmission means that we have to assume that these new cases pose a significant risk,' said Director of Public Health Nicola Brink.
‘A lockdown is the most effective and efficient way to prevent onward transmission of Covid-19. We know from other jurisdictions around the world that hard and strict measures introduced quickly generate the best results and that is our focus - to try and get this under control and us back to the normality we have all enjoyed as quickly as possible.’
Since the 20 June the island has been free of all restrictions internally, with the only controls being applied to travel.
As of the 8th January only essential travel has been allowed into the island as concerns mounted about the growing number of cases in the UK, Jersey and Europe, and the emergence of a new more easily transmissible strain.
Yesterday there were only six active cases of the virus, all linked to travel.
This morning the States urged people to cancel events and gatherings and to socially distance.
The hospital and care homes closed to visitors.
Organisations immediately reacted, with the Guernsey Dance and Upload festivals among those cancelled.
All sports was also called off.
Beau Sejour, where the community vaccination centre will open on Monday, was shut.
The States was forced to urge people not to panic buy after large queues formed at all island supermarkets.
As of 17th January, 4,640 vaccine doses had been given, 519 of those were second doses.
Since the start of the year 5,353 people have arrived in Guernsey - 4,232 from England, 114 from France and 128 from Jersey.
There were 10.35 infections per 1,000 travellers on a 14 day rolling average.