If nobody can hear you then check to see if you are muted. This might be caused by a physical control on your microphone, by the mute button next to your profile icon at the bottom of the channels list, or by your operating system's sound controls.

If you aren't muted, it could be a connection issue. Briefly leave the channel and then rejoin it.

Finally, check your audio configuration to check that you have set the input to the correct microphone and that Discord is picking up your voice.