Why did the microphone of my phone just stop working?

If the microphones on your iPhone, iPad, and iPod touch aren't working

If the sound is unclear during phone calls or FaceTime calls, or if Siri is unable to detect your voice, learn what to do.

With your iOS device, you might experience one of these issues when you make a call, use FaceTime or Siri, or play recorded audio:

• People can't hear you during a phone or FaceTime call.
• You sound unclear to others during a phone or FaceTime call.
• Siri says, "I didn't get that" or doesn't hear your request or hears only part of it.
• When playing back recorded audio, the sound you hear is unclear, or you don’t hear sound. 
• You're unable to change audio sources during a call.

If the microphones on your iPhone, iPad, and iPod touch aren't ...

https://support.apple.com/en-us/HT203792