If your headset connects through audio jacks - the mono/stereo (green) and mic (red) jacks need to both be plugged in.
If your headset connects through USB or Bluetooth- you need to have the correct drivers for your particular device installed. Alternately, you could try plugging them into a different USB port before booting the machine.
(Always prefer the rear I/O ports on the motherboard instead of any ports built into your PC chassis/accessories when troubleshooting).
If you're using motherboard-integrated audio you may need to enable it with a BIOS setting.
If you're using an audio card, you may need to enable the PCIe/PCI slot with a BIOS setting.
You may need to install a driver for your audio device, if your operating system hasn't done so automatically.
Have you checked through all your control panel (and Realtek Audio) settings? Your system might be configured with "speakers" instead of "headphones/headset/microphone" as your primary audio device, or your microphone might be entirely disabled/muted by default.
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