thanks for answering.
@JimDrew: I only have one power socket in my room, no problems with other stuff so far. I'll try a power cord from anther room. But this can't be my solution.
@bernouilli: Why not? The DE10 manual says, I can see the board as storage device when connected to a PC.
Yes, you're right, it can be connected, I was just wondering why did you get the idea to connect it toi the pc to see if it solves your problem.
Have you tried another hdmi cable ?
When I first tried my de10, I couldn't get a picture. I had to change the hdmi cable to have a picture, althought the hdmi cable was fine with other devices. I should find this cable and see if connecting the board to the pc solves the problem.