If Beoplay HX are connected to a USB-C cable, disconnect the cable, press and hold the power button, then reconnect the cable and try again.
If connected to Bluetooth, your Beoplay HX 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 Bluetooth devices.
If your Beoplay HX are still unresponsive, find out how to reset them here