Not wanting to be picky, but you didn’t say that. You didn’t mention that you were using a USB device.
I assume from your reply that you are do have a USB audio device plugged in. Is that correct?
If so, what sort of USB device is it?
Is audio restored if you shut down the computer, remove the USB audio device and restart the computer?
Do you have any sound in other programs, or is all the audio dead?
Are you using external computer speakers, headphones or built-in speakers?
Right-click the speaker icon by the system clock > Open Volume Mixer. Make sure the Audacity volume slider in Volume Mixer is turned up. Make sure the output volume slider is turned up in Audacity too: