I have had my DELL U2713HM ULTRASHARP 27" for about half a year. There have been som problems with it, it has gone on and off without any reason. And now it has crashed completely. It's all black, and nothing works at all. We had a small fall out in the electricity (one minute), and when the power came back, the monitor stayed off, when all other devices started up again. I disconnected all cables and connected them again, but not even the start button reacts. What do I do?