I think the actual problem was that someone typed "hostname " (that is "hostname" and 2 blanks), which did set the hostname to " " (a blank). Yeah sure changing it back may work (it didn't for the colleagues that told me that).

Here is another one: I once shut down a server because I wanted to know how to shutdown & reboot. I typed "shutdown -h" (I thought that would display help-pages :)

It was the option to "halt" immediately.

We needed to open a ticket to start the machine up again.