Had one of those problems today. The server had to be restarted. The client pressed the power button and held it for 5 seconds, because there was nothing on the screen. Then they turned the server on again. Nothing. Tried it a few times with no luck.
I got to the site and tried it too. No luck. Finally, I pulled the power cord out of the power supply. Plugged it back in after 10 seconds, and up came the server. Why? I don’t know…