Audacity is not recording: how to find which link in the chain broke

This is the cause more often than everything else combined. Look at the device toolbar at the top of the Audacity window and read what is selected as the recording device.

If it names a headset you are not wearing, an interface that is unplugged, or a virtual device such as BlackHole left over from a previous session, Audacity is recording perfectly from a microphone nobody is near.

Set it to the microphone you are actually using. If the device you want is not listed, use Transport, Rescan Audio Devices, which picks up anything connected after Audacity started.

Also check the channel count next to it. A mono microphone set to record two channels sometimes produces a silent second track that looks like a failure.

On macOS, open System Settings, Privacy and Security, Microphone, and confirm Audacity is listed and switched on. If it is missing entirely, the permission was never requested, which happens when Audacity was installed by copying rather than through an installer.

Toggling it off and on and restarting Audacity fixes cases where the setting looks correct but nothing arrives.

On Windows, Settings, Privacy and Security, Microphone, and check both that microphone access is on generally and that desktop apps are allowed. That second toggle is separate and is the one people miss.

Permissions can be revoked by a system update, so this is worth checking even if recording worked last month.

The Audio Host dropdown, the leftmost item in the device toolbar, changes which system audio layer Audacity uses. On Windows, switching between MME, Windows DirectSound and Windows WASAPI resolves a surprising number of cases where one host cannot see a device the others can.

Note that WASAPI loopback devices record playback, not microphones. If you chose one of those by accident and nothing is playing, you correctly get silence.

Sample rate mismatch is the other one. If your project rate, shown at the bottom left, does not match what the device supports, recording can fail silently. Set both to 44,100 Hz or both to 48,000 Hz.

Video conferencing apps hold on to the microphone after a call ends, and some streaming and recording tools claim exclusive access. Audacity then finds the device present but unusable.

Quit everything that touches audio, including menu bar and system tray items, and try again. If that fixes it, the culprit is usually whichever call app you used most recently.

Under Audacity's Settings and Recording there is an option for exclusive device access on some hosts. Turning that off lets Audacity share the microphone rather than demanding it.

If Audacity records a line that is technically not flat but almost, the input level is too low rather than absent. Raise the recording slider in Audacity, and check the gain knob on the microphone or interface itself.

Many USB microphones have a physical mute button that is easy to knock, and some have a light that is the same colour in both states. Check it by hand rather than by looking.

Condenser microphones on an XLR interface need phantom power, the 48V switch. Without it they produce nothing at all, and this is a very common cause of total silence from a microphone that works elsewhere.

Finally, test the microphone in another application, your operating system's own sound settings will show a level meter. If it fails there too, the problem is the hardware or the cable rather than Audacity.