if you have installed a hard disk on it, try removing that and booting without the hard disk. or if you can telnet to the box, make sure that boot from flash is enabled or delete recordings.epl from hdd/movie/ since it might be corrupted.
if that does not boot the device, turn off the device. now hold the lower button (or all the buttons) on the front panel, power up, keep on pressing until you see the logo for 10 seconds (or the orange light comes on since you cannot see anything on the tv), release and hold the button again for a hard reset of your box. all this does is a cold boot for your dreambox.
if all else fails, JTAG with cas2 or something else.
if you can specify which procedure you were doing prior to this failure, that would help too. but i suspect you were flashing something to the box.