Faint sound from the speakers
Most speakers are mains powered (either plugged directly into a wall socket, or maybe using a 12volt adaptor), and inside the speakers will use a small amplifier circuit. These circuits are prone to often going faulty and may lose one speaker or both, so instead the speaker will then only play the sound as it gets it from the PC (or hifi etc.).
Audio to speakers is not sent in a 'loud' form, but is instead just a very high quality weak signal that must be amplified to be heard. Typically a PC sound output may be just .5 to 1.5 volts only, just enough to work a set of headphones but nothing much more.
I suggest checking your speakers on another item, such as a CD player or hifi that definatly works, and if its the same, its most likely the 'amp' gone faulty. It's not really worth fixing as they usually only break in cheaper types of speakers - so its as easy to just buy another set.
If they work OK on other devices, maybe check these things:
Is the volume up on the computer ? Check on the volume icon on the taskbar, and also if its a laptop check its not got a volume dial that has been accidentaly turned down/off.
Test the computer with other speakers, to just narrow down the fault.
Some PC's have several output sockets at the rear (or front), so make sure you have it plugged in to the correct socket - even if you have used it before, some PC's software can actually change what comes from each socket, so your MIC socket could now have become your SPEAKER socket, or other settings may have been changed by other software.
Check your operating system is not using a 4 speaker setup instead, and sending the sound to non-existent rear speakers/woofers/tweaters. My own PC has 8 outputs, so I make sure nothing alters my settings or i'd never hear it again ;)
Its also possible for the PC audio chip to go faulty, but that is very very rare. If nothing else works & the speakers are known working ones, you may have to try a new sound card.
Remember that plugging the standard 3.5mm stereo jack plug into the wrong sound card socket will not cause harm, you may just hear a loud 'THUMP' or click as the power goes across.
One last thing is, try reinstalling your audio drivers. PC operating systems can lose driver functions and do any possible thing you can imagine. It may be as simple as a reinstall to wake up the audio card again.