lemmy.ml's server is undersized and has been having trouble with the growth. They apparently can't upgrade their instance on the tier that they are on without an extended outage, so they're kind of stuck. (at least that's what I heard). I still have been able to see their content, but I haven't been able to join communitites and whatnot.

Give it a few days and I'm guessing the communities on lemmy.ml will be back to being usable or the community will naturally migrate to a different instance.