Step 1: test outside the app you were using
Open the mic test and allow access. A moving level meter means the microphone and the operating system are fine, and the fault is inside the application that failed — usually a device selection or a mute state inside that app. A flat meter narrows the problem to the system, and the remaining steps apply.
Step 2: permission and the right device
Browsers and operating systems both have per-app microphone permissions, and a previously denied prompt is remembered silently. Then check the device list: laptops with a headset jack and a USB microphone frequently keep using the internal mic while you speak into the other one. If the level meter responds only when you tap the laptop, that is exactly what happened.
Step 3: mutes and levels in three places
There are usually three independent mute points: a physical switch on the headset or the mic, the operating system's input level and mute for that device, and the volume slider inside the calling application. All three can be on at once. Set the system input level to about three quarters, speak normally, and watch the meter while you adjust — this is where the level meter is genuinely better than guessing.
Step 4: exclusive access and drivers
On Windows, one application can take exclusive control of an input device: a conferencing client still running in the tray, or a voice-chat app, will hold the microphone even after you close its window. Quit them from the tray, or disable exclusive mode for the device in the sound settings. If the microphone is a USB device that has never worked, try another port (front and back are different controllers), another cable, and check the vendor's driver.