Check that the volume on your speakers or headphones is up.

Check which devices are connected. Other devices that have built in microphones may take priority over your microphone. These can be located in devices such as Laptops, Webcams, Connected Phones, Controllers, Monitors etc. and may need to be disabled temporarily.

Check that the microphone is connected. For a Bluetooth headset, make sure it's paired correctly and that no other devices are using it. If it's a mic with a USB connector, make sure it is properly connected to the USB socket. If it's not USB, it's normally plugged into the pink socket on your computer.

Check that your microphone is not muted. Sometimes the mic has a mute button on it or on the wire that is connected to it.

Check that the volume on the microphone is up.

Check that microphone levels are up. See screenshots below.