Customers will be unable to access the site from 5pm on Wednesday 7 November until 5am on Thursday 8 November, meaning they won’t be able to check-in for flights online.
Europe’s biggest airline is advising travellers with flights on Wednesday and Thursday to check-in on Tuesday 6 November; passengers should have already been contacted via email and text.
Checking changes to existing flights will also be impossible during the system shutdown.
Collected and summarized from the source below by Minh Pham https://www.independent.co.uk/travel/news-and-advice/ryanair-website-down-12-hours-november-check-in-flights-upgrade-fees-a8618681.html