Typically, when we say a server is down, what we mean is that the program that is designed to respond to requests from the network is not doing so. There are a number of reasons that could happen, and each of them call for a different solution. Here are some example:
1. The power to the computer it runs on was cut. There are many reasons this could happen, including but not limited to people pulling the wrong plug while doing maintenance, cleaning, etc. In that case, you have to restore power.
2. The network connection stopped working. This could be because of a physical cable that got unplugged, or because some piece of network equipment failed. Network equipment can overheat. Sometimes there are subtle bugs in the software it runs that only show up every once in a while, and the network equipment needs to be restarted. Sometimes, some totally unrelated system using the same network can go haywire and flood the network with too much traffic, which could slow down everything else and sometimes even trigger problems in the software or configuration that aren't usually noticed.
3. The computer is connected to power and the network, but for some reason it is not actually on. Maybe the power was cut earlier, and the computer is not configured to turn back on when the power is restored. Maybe
