Here are our recommendations to narrow this issue:
1. Test the system out of the chassis on a minimal configuration. If you have a cooling fan connect it to the fan header to check if you are having any fan activity.
2. Remove the RTC (Real Time Clock) battery and leave it out for 20 minutes. After this period of time, insert the battery back in and turn the system back on; if it displays POST messages, go into BIOS Setup Utility by pressing <F2> during the boot process. Once in the BIOS setup utility, press <F9> to restore BIOS default settings. Reset any customized BIOS settings. Clear all DMI event logs, which are located in the Advanced/Event Log Configuration section of the BIOS Setup utility. Press <F10> to save the new settings and reboot the system.
If the issue still persists, check your warranty options to replace the board.
thanks for your fast answer.
1. I have checked it with minimal configuration. And I have fan activity. The backlight of my keyboard is also active.
2. I took the battery out for 20 minutes. The screen remains still black (I checked the connection to my monitor but it gets no signal). I now tested it only on the DVI-port. Could it be different on the HDMI out?