Hi Valentin!
Usually this indicated a problem with the HDD.
Please try another HDD completely formatted and put it in. It should at least display an error.
If this is the case, it's the HDD which is broken. Then indeed use a known working (or new) HDD formatted with the Xbox HDD script.
It's also possible that the SATA cable is broken or the power connector, please check those out too.
Off course it's possible that something else died on the motherboard. If the above all failed, then check the motherboard again...