Note: This is a fairly common problem with several causes and resolutions. If you come across a cause and resolution that is not documented here, please add it.
No sound is coming from your computer, even though audio was working previously.
There is a red X over the speaker icon at the bottom right of the screen in the system tray. When you hover your mouse over the speaker icon, you see the following message:
No speakers or headphones are plugged in.
The Playing Audio troubleshooter does not detect any problems and in Device Manager the audio device shows up correctly with no errors. Though there are many types of integrated audio, a common one is the Realtek HD audio device which Windows detects as an High Definition Audio Device in Device Manager.
The onboard audio is disabled in the BIOS. Even if you did not manually disable the onboard audio, it may have been disabled automatically when the BIOS detected a new audio device was installed. For example, if you are using onboard audio and you install a new video card, some video cards now come with audio devices built-in, often that output to HDMI. The BIOS detects the new audio device, which is actually part of the video card, and disables the onboard audio automatically. Since there is actually still an audio device present and detected by Windows, the error message reflects that you do not have speakers or headphones plugged in because you have nothing plugged into the HDMI port on the video card.
Restart the computer and enter the BIOS setup (generally by pressing the F10 or F2 function keys at the very beginning of the boot process). In the BIOS setup, find the section for integrated devices and enable the integrated audio (onboard audio). Restart the computer and the audio will work again.