If your instance goes down, you lose all you posts. vlemmy.net was an instance and they just vanished recently. lemmy.fmhy.ml just disappeared yesterday, I couldn’t find much info besides a post vaguely mentioning that their domain name got seized by the Mali government (idk why). Besides being shut down, also due to federation issues. beehaw.org has defederated from lemmy.world and sh.itjust.works, meaning you can’t really interact with each other for the most part. Some instances like lemmygrad.ml and exploding-heads.com are disliked by many instance owners due to politics and got defederated so they are sorta on their own island.
Edit: Well… techically, your posts aren’t lost, you can still find them in other instances. But your images are gone since images do not get federated (unless someone made an archive) and you lost the account and any direct messages, and you have to manually check if you want to see any comments to a post that you posted before it got shut down. Just a huge inconvenience.