|I'm assuming Device Manager reports the sound device as working properly. |
On most laptops, when you plug in headphones (or external speakers) into the headphones jack, the laptop speakers are disabled from outputting sound, by means of a mechanical part built into the headphones jack that makes an electrical connection when a plug is plugged in. Sometimes that mechanical feature does not work properly, more likely if you have used the headphones jack a lot, such that when you unplug the plug, one or both built into the laptop speakers do not output sound as they should.
Try plugging in headphones (or external speakers) into the headphones jack, wiggling the plug, then pulling the plug out. If that doesn't help you may need to replace the headphones jack.