Moin,
if the microphone, cable and plug is fine (sure? because this indeed sounds like a hardware/connection problem), have you checked the device manager for red or yellow exclamation marks? How does the socket at the rear of the PC look like, sometimes there can be dust which becomes compressed over time.
Then check the Audio manager of the audio card or built-in sound chip (mostly Realtek these days). If it is a laptop there is sometimes a button to cut off the mic for direct input.
Sorry if you tried all this.
Good luck
