|The headphones jack has a mechanical switch inside of it. It's supposed to disable the sound from going to the speakers built into the laptop whenever you plug a plug (on a cord) into the headphones jack.|
If you have plugged into the headphones jack a lot, it's common for that mechanical switch to malfunction. You could try plugging a plug into the headphones jack, wiggling it, then pulling it out, that may get one or both speakers working, but the only real solution is to replace the headphones jack - that's frequently replaced item done by laptop repair places.
OR - you could make do by plugging amplified speakers into the headphones jack.