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If your computer is a laptop or netbook...
The headphones jack on laptops has a tiny mechanical electrical switch inside of it that disables the built in speakers from producing sound when a plug is plugged into the headphones jack.
If you have used the headphones jack a lot, it's common for that mechanical switch to malfunction. If you insert the plug for the speakers into that jack and wiggle it, that may get both speakers to produce sound again, but the only lasting solution is to replace the headphones jack - laptop repair places do that frequently.
If your computer is a desktop computer, if you are plugging the jack for the speakers into a jack built into the computer case, not a jack built into the mboard on the back of the computer, you have a wiring problem between the case jack and the mboard.