Pixhawk Firmware does not boot


I am using a pixhawk, which does not boot.
I am still able to upload a new firmware to it, but the problem stays the same.
It is similar to the one described in this post: http://ardupilot.com/forum/viewtopic.php?f=21&t=14410

The problem first occurred, when I uploaded new, custom firmware to it.
I also flashed arducopter and arduplane on the device, but the problem stayed the same.

I also tried to flash a new bootloader on it, but since I can not connect to the pixhawk, it is not possible to do it from the sd card. The software used to upload it via DFUse wasn’t able to recognize the board.

Do you have any ideas why this problem occurred and how to solve it?