Voice control

Have you confirmed that your microphone is picking up sound? On my iMac when I speak the microphone icon acts like a sound level meter. Does your microphone icon change as you speak to indicate that it is working correctly?

If the microphone icon doesn't change, then I suggest opening System Preferences/Sound, then click on the Input tab to confirm that you've selected the Built-In Microphone as shown below. Check that the Input Volume slider is set to an appropriate level, then speak into the microphone while monitoring the Input Level meter to make sure that the microphone is working. If the microphone doesn't seem to work then reset the NVRAM and try again. If you continue to experience problems you may want to contact Apple or your service provider for further troubleshooting.

I'm glad to hear it's working after the reboot. One of the reason's I mentioned doing the NVRAM reset in my response is that it forces you to restart your iMac. I figured there is no need to call it out separately since the NVRAM reset would take care of it. Although the cause remains a mystery, the important part is that your iMac is working.