my Dell All-In-One PC XPS One 2710 (manufactured in 2014) suddenly won't boot anymore and is stuck at the Dell Logo Screen.

I can't enter the BIOS or the boot menu. When i hit F2 or F12 a loading bar appears and a message to press F12 appears, but no matter what key i press the loading bar disappears and nothing happens anymore.

I already have tried the following:

* Reset the BIOS with the Jummper
* Reset the BIOS by removing the CMOS-Battery and pressing the power button for 10 seconds
* Remove all Drives (HDD, SSD and BluRay) and boot
* Formatting the harddrive
* Trying to boot with 1 of 2 RAM-modules (each one alternating)
* Replacing the RAM with 100% working RAM from a notebook
* Replacing the CPU
* Connected the harddrives to another computer and check/repair the filesystems

Nothing worked for me. 

I've tried to contact Dell for support but they told me the pc is already to old and they can't do anything to help me.

I also tried local computer repair shops but noone is willing to help because the hardware is so "special".

Does anyone have more ideas?