GeoServer is a great software, and core developers hit bugs every day, enhance existing functions, and deliver new capabilities. Despite all of this, and the careful configuration of your site, it is just a matter of time before you will encounter a failure that prevents your GeoServer from delivering maps. In the simplest of cases, it will only affect some specific requests; more often it will halt it for a while, and sometimes you will need to restart it to get it working again.
It happens to almost all the software applications that you will have worked with, either proprietary or open source, free of charge or very expensive. Avoiding faults is out of your control, but you should learn how you can manage avoiding service interruption.
A high availability or fault tolerant configuration is what you need. Indeed, this a very common approach in software deployment and what you will learn here is best practice for any kind of software service, not only for the map...