We had been waiting at 0.18.5 for a while because 0.19 came with some federation bugs (and obviously it would be extremely embarrassing if this instance had issues in that department).
The biggest changes for users are a new scaled sorting method that should prioritize posts from smaller communities (no more Risa dominating the entire feed), the ability for users to block whole instances, and the ability to import/export accounts with other instances (or just back up your account).
Also I'd like to apologize for the downtime; we expected this to be a simple update taking only a few minutes, so I didn't post an alert in advance, but then were some issues that caused us to go offline for several hours.
Lastly, after the update I found I had to clear my browser cache/cookies in order to log in, similarly, users using apps have been reporting they needed to remove and re-add their accounts. You may find you need to do that too!
EDIT: The app Liftoff has a bug preventing login on any Lemmy instance over v0.19.0
EDIT 2: We've updated to 0.19.3, mostly just bugfixes in this one.
Hopefully people who browse lemmy via browser are finding a way to read this page. This post was failing to load for me even while logged out for some reason. Luckily I saw it from a mobile app and knew to clear cache/cookies, because otherwise I might have just assumed it was down.
We also have a Mastodon PSA up - unfortunately, with the post being difficult to access, there's not too much else we can do - I ran into the same issue, myself.