They were down but aren’t. This is going to happen from time to time for reasons, but most importantly (and this is not an advert or endorsement for centralized services like reddit):

• these instances are run by small teams, maybe even one person per instance. By “run by” I mean the admins who can actually host and support the hosting environment of the instance, not moderators though that’s an important task too.
• At reddit or other for-profit companies, multiple teams of people monitor multiple data centers worth of servers, have 24/7 tech support crew, dashboards, alarms, alerts, escallation proceedures drafted by other teams, people they can escallate problems to including usually a decent sized team at the physical datacenter due to the amount of servers they buy because of what they can afford based off advertising income because the site is popular enough, which is why it’s much more rare to see these services go down.

But so many things can and do fail, including:

• updates (dependencies, breaking updates, “this should just have worked but it didn’t, why?!”)
• server issues (too many memes and now the disk has runeth over)
• one server that gets overloaded or is in a data center that has a network failure, or a hardware failure on the server where the virtual server is hosted
• account got hacked
• 0 day exploit targeted directly at this server
• DoS or DDoS attack
• Admin has a day job that they need to do to keep the lights on at home and at the lemmy instance and has to do their day job work.

Speaking from experience, but not with lemmy in particular.