Silence is usually a routing problem, not a hardware one — the sound is going somewhere you're not looking, or a driver has hiccuped. Work from the simplest explanation (wrong device, mute) to the least simple (driver, hardware).
Check the output device and mute
Click the speaker icon in the taskbar (or the menu bar on a Mac). First, make sure the master slider isn't at zero and the mute isn't engaged. Then, if you see a list of output devices, select the one you actually want — built-in speakers, your headphones, or an external set. It's shockingly common for Windows to route audio to a headset you've unplugged or a TV that's off.
Check the app's own volume
Many apps keep their own volume separate from the system. In the speaker fly-out, open the volume mixer (or the app's own settings) and make sure the specific app isn't muted or at zero.
Test with a different device
Plug in a different pair of headphones or speakers. If they work, the original output (or its cable) is the problem. If nothing works, the issue is inside the PC.
Restart the audio service / driver
On Windows, press Win+R, type services.msc, find Windows Audio, and restart it. If that doesn't help, open Device Manager → Sound, video and game controllers, right-click your audio device → Update driver, or Uninstall and reboot to let Windows reinstall it. On a Mac, quit and relaunch the app, or toggle AirPlay output off.
Check for a stuck Bluetooth device
If you have a Bluetooth speaker or headset paired, it may be 'connected' and siphoning the audio. Disconnect it temporarily (Bluetooth settings → disconnect) and the sound should return to your built-in speakers.
Rule of thumb: one device silent → that device or its cable. all devices silent → the output selection, the driver, or the audio service. Start at the speaker icon and work outward.