Try right clicking the sound icon in the notification area and select playback devices. Once the pane opens connect the headphones. They should now show up in the list. Click on the headphones and make default.
Playback Devices fixed the problem. HEADPHONES does not show up in the list, only speakers, hdmi, and s/pdif. However, the jack started producing sound. I don't understand, but the problem is fixed. I hope I remember this next time the problem occurs.
Thanks to everyone for their responses.
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