Life happens, it's not that big a deal, and most followers will understand. If some people unsub, try to shrug it off! It is not the end of the world, and when you start regular updates again, new people will jump on board. And lots of people will likely be enthusiastic about your return. Just give them a heads up that you were gone for health reasons, and you'll be returning once you build your buffer back up.
Maybe start on the buffer first, and see how working again feels to you. You don't need to start over from scratch unless you'd prefer to do that. But choose whatever path feels comfortable.
Seconding others suggestion that you schedule breaks in the future so you have time off, and can build your buffer back up. Missing a few weeks or even months is not a big deal when you're working on a passion project for free. I take a minimum of a month off between chapters, but it usually stretches to two, and a week off as a mid chapter break. I've never felt like it turns off my readers. Generally people are understanding and supportive. =)