If it happened right now, we'd need to go make new accounts on a new instance to keep accessing content. Of course, content hosted on the disappeared instance would also be gone, which is another reason spreading everyone out onto smaller instances is a good idea to avoid one central server holding 90% of the good content.

What we have on Mastodon is an easy way to migrate account from one instance to another, so I'm hopeful a similar feature will be added here in due time once the devs have everything a bit more stable. The added complication with Lemmy of course is that we'd also need a way to migrate communities or at least the content thereof, but I've seen various discussions of how that could work so it is something that they're thinking about already.

Ultimately, don't get permanently attached to content you put out on the internet, because if this sounds terrifyingly precarious to you you probably weren't online in the early 2000s when that's just sort of how things worked. Somehow we thrived anyway!