Just announced. The Ryanair website is shutting down for an upgrade from for 12 hours, from 17:00hrs (UK time / GMT) on Wednesday 7th November, until 05:00hrs on Thursday 8th November.
During his time not only can no new tickets be bought, but also no on-line check-in.
"The BBC has asked Ryanair if it might waive airport fees for passengers who leave it too late to check-in early, but has yet to receive a reply." If you are flying, make sure you do your online check-in before the outage.
BBC article: https://www.bbc.com/news/business-46100074
Ryanair webpage: https://www.ryanair.com/gb/en/travel-updates/articles/reservations-system-upgrade