See the details here
IS the list unpublished?
View attachment 20894
Did you change something? If not it seems something has changed on your server.
Maybe your host changed php version, changed security settings, your site got hacked...
This doesn't change anything on your server/site, you can run J!3 "as it is" as long as you like resp. as long as you can justify it for safety reasons (assuming your server doesn't change php and/or database versions etc)J3 no longer being supported