When a web server is not functioning correctly it alerts an administrator, so that they can begin to analyze the problem in detail.
Such problems can manifest themselves when web pages, databases or email are being slow or become unavailable.
In most cases the server identifies the particular problem itself and has some automatic processes for dealing with it, but it may still take an administrator to discover the exact cause and to minimize or remove whatever led to the problem.
If a significant problem with hardware, file corruption or security is identified then the message may later change to reflect what the administrator has discovered.
Problems are usually identified and fixed quite quickly, but occasionally they take more in-depth investigation.