PX4 flash fail after flashing non-PX4 firmware

I can not flash the PX4 anymore, neither with USB nor with dronecode probe.
First the Pixhawk worked fine.
Then I flashed a non px4 application (just a simple stm32 project)
Since then the pixhawk-USB-port is gone (No /dev/ttyACM2) and the pixhawk-LEDs are off.
Furthermore I fail in connecting with the droncode probe to the pixhawk:
(gdb) mon swdp_scan
does not find anything.

Maybe I have overwritten the bootloader. But how can I upload one again if flashing does not work?

Any suggestions or ideas?

Thank you

Here is one suggestion - use STM DFU to reflash the boot loader.