Hackers normally don't kill web sites (unless they have specific interests in keeping the site dead) but instead tries to either publish a message or steal the server bandwidth by using it as a distribution server for spam, viruses, illegal software etc.

But a server can be down for a lot of reasons.
- failed PSU
- local problem at the ISP
- a critical process that is hanging
- damaged network cable
- disk partition getting full
- accidental shutdown of the machine
- blue screen of death (if running M$ Windoze)
- ...