If Beoplay H9 3rd Gen are connected to a USB-C cable, disconnect, then reconnect the cable and try again.
If connected by Bluetooth, your Beoplay H9 3rd Gen have a Bluetooth range of up to 10 meters (in line of sight). However, Bluetooth connectivity might be affected by other active Bluetooth devices in the nearby area (e.g., mouse, keyboard, mobile phone, laptops or 2.4 GHz routers). Try to keep the headphones away from other active devices.
If your Beoplay H9 3rd Gen are still unresponsive, find out how to reset them here.