At a guess I'd say the site tripped over the 150,000 tile requests per month limit that you can do with a free account.The first non-free tier is £95/month, probably too much given the relatively few subscriptions and other costs.Here's OCM's pricing tier page: https://www.thunderforest.com/pricing/
Has the OS map option been permanently removed?I noticed it had hit a daily tile limit the other day, but the option now seems to be completely gone.