A microphone that is detected but picks up nothing is usually muted, blocked by privacy settings, or set to the wrong input device. Windows has separate permission, device, and level controls for the microphone, and a problem in any one of them silences it while everything else looks normal.
Fix it, in order
Check the app has microphone permission. Open Settings, Privacy and security, Microphone, and make sure Microphone access and Let apps access your microphone are both on. Then check the individual app in the list below, since a denied app hears nothing even when the system microphone works.
Select the right input device. Open Settings, System, Sound, and under Input, pick the microphone you actually want. Windows often defaults to a webcam mic or a headset mic you are not using, and the wrong input looks exactly like a dead microphone.
Test the input level. In the same Sound settings, speak into the microphone and watch the Input volume bar. If the bar does not move, the mic is muted or the wrong device is selected. If it moves but apps hear nothing, the app permission above is the problem.
Unmute and raise the level. Select the microphone in Sound settings, then check that the Input volume slider is up and the mute toggle is off. In the older Sound control panel, open the Recording tab, double-click the mic, and check the Levels tab as well.
Run the microphone troubleshooter. Open Settings, System, Troubleshoot, Other troubleshooters, and select Run next to Recording Audio. It checks the device, the levels, and the privacy settings in one pass.
Disable audio enhancements and exclusive mode. In the microphone properties, turn off audio enhancements, and on the Advanced tab uncheck Allow applications to take exclusive control of this device. An app holding exclusive control can block every other app from the mic.
Reinstall the microphone driver. In Device Manager, expand Audio inputs and outputs, right-click the microphone, select Uninstall device, and restart. For a USB microphone, also try a different USB port, since a port that works for storage can still fail an audio device.
Test the mic on another device. Plug the microphone or headset into a phone or another PC. If it is silent there too, the microphone itself has failed, and no Windows setting will bring it back.
Browsers ask for microphone permission separately from Windows. If the mic works in a voice recorder but not in a browser call, check the browser's own site permission for the microphone, not Windows.
If it still happens
If the input bar moves in Settings but no app hears the microphone, an app is holding exclusive control, or a privacy tool is blocking access. Close voice apps one by one, and check any security software with a microphone guard. If the bar never moves on any device, the audio driver is the next suspect, so reinstall it from the PC maker's site.
Common questions
Why does my mic work in one app but not another? The app that works has permission and the app that does not was denied, or the working app holds exclusive control. Check Settings, Privacy and security, Microphone, and the exclusive mode setting in the mic properties.
The mic is detected but the input bar never moves. What is wrong? The mic is muted, the level is at zero, or the wrong input device is selected. Check the Input volume slider and the mute toggle in Sound settings first, then test the mic on another device.
My headset mic stopped working but the headphones still play sound. Why? Headset microphones use a separate connection from the earphones. On a wired headset, a loose or partially inserted plug is the usual cause, and on Bluetooth the Hands-Free profile has to be active for the mic to work at all.