Headphones Not Detected, Fix Wired and Wireless Headphones
Headphones that Windows does not detect fall into two groups: wired headphones the jack does not recognize, and Bluetooth headphones that paired but never became a playback device. The fix depends on which kind you have, so start with the section that matches your headphones.
Fix it, in order
Push the plug in fully and try another jack. A wired plug that is not fully seated will carry no signal at all. On a laptop, try the only jack available, and on a desktop try the rear jack instead of the front panel, since front panel jacks fail more often.
Test the headphones on another device. Plug them into a phone. If they are silent there too, the headphones or the cable have failed, and no Windows setting will fix them.
Show disabled devices. Open Settings, System, Sound, More sound settings, then the Playback tab. Right-click the empty space, tick Show Disabled Devices, and if the headphones appear greyed out, right-click them and select Enable.
Set the headphones as the default. In the Playback tab, right-click the headphones and select Set as Default Device. Then click the speaker icon on the taskbar, then the arrow next to the volume slider, and select the headphones there as well.
For Bluetooth headphones, remove and pair again. In Settings, Bluetooth and devices, Devices, select the three dots next to the headphones, choose Remove device, then put the headphones in pairing mode and add them fresh. A stale pairing often connects without ever registering as a playback device.
Reinstall the audio driver. In Device Manager, expand Sound, video and game controllers, right-click the audio device, select Uninstall device, and restart. For Bluetooth headphones, also update the Bluetooth adapter driver under the Bluetooth category.
Check the audio jack setting. Some audio software asks what you plugged in, headphones or speakers, when a jack is used. If that prompt was dismissed or answered wrong, the jack may be configured for the wrong device, so open the audio control app from the PC maker and check the jack setting.
Laptop speakers keep playing when the headphones are not the default output, which looks like the headphones are broken. Always check the taskbar volume menu first, since it shows exactly where Windows is sending sound right now.
If it still happens
If wired headphones work on a phone but no jack on the PC detects them, the audio driver or the jack sensing is at fault, so reinstall the audio driver from the PC maker's site. If Bluetooth headphones pair but never appear as a playback device on any PC, reset the headphones to factory settings using the manual's procedure.
Common questions
Why do the laptop speakers keep playing when my headphones are plugged in? Windows is still sending sound to the speakers as the default output. Select the headphones in the taskbar volume menu, or set them as the default in the Playback tab.
My headphones appear in Bluetooth but not in sound settings. Why? The pairing connected but the audio profile did not start. Remove the device and pair it again, and check that the Bluetooth Audio Gateway Service is running in services.msc.
Headphones are detected but very quiet. Is that the same problem? No, that is a volume or enhancement issue, not detection. Check the volume in the taskbar and in the app, turn off audio enhancements in the device properties, and test the headphones on another device.