|The headphones jack on laptops and netbooks 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 properly, but the only real solution is to replace the headphones jack - that's an item frequently replaced by laptop repair places.
OR - you could make do by plugging amplified speakers into the headphones jack.
Other than that,
- RIGHT click on the speaker icon in your taskbar and check your volumes in the Windows mixer settings - e.g.for CD sound.
- go to Control Panel - Classic view - click on the icon for your brand of sound adapter, check the settings in that. E.g. some sound adapters have a setting for Environmental sound or similar that can be set such that the sound is as if you are in a large space