One day my PC didn't start up in Windows as usual, but started into the Bios.

The Reason was that my HDD was not detected anymore. 

I first suspected the HDD or the cable to be at fault and replaced them with working ones but had no luck.

Updated the Bios and checked all the other Sata Ports no luck as well.

I reset the Bios, cleared the Cmos etc.

Bios is still working so I doubt my Mainboard is dead, but I highly doubt that all my Sata Ports died over 1 night.

Does someone have a solution what I could do else?

Make sure your SATA controllers are enabled in BIOS under "Advanced\Storage configuration" then set them to IDE mode for compatibility.

Also be sure that the SATA cable used is SATA3 compliant, I have noticed that if you plug a SATA1 standard cable into a SATA3 port it can cause the system to fail to post or simply not detect the drive connected. I say this because connecting the same drive and cable to a SATA1 or 2 port on another board allows the device to be detected and function. In other words, if the cable was not compatible with your board it may still have worked in another system if that system uses SATA2 or older.

Does the drive spin up and initialize in the system? If you put your ear to it you should hear it begin to spin followed by some activity.

The system was working fine until the hdd stopped getting detected.

So I can count out that the cables or Bios settings are wrong (double checked).

The HDD spins and I can actually feel when it is under power. Also tested on another PC to check if its faulty. Not the case here.

If the board was a recent purchase then you can RMA it, if not you will need to replace it.