Sound Stopped Working

Hi,

I turned on my computer this morning, an hour or so after my brother was using it, only to notice the speakers green cable thing was plugged in the front of my computer, in the headphones slot. I thought 'that's strange'. Once logged on, I noticed my brother had left me a message on the PC saying the sound isn't working when plugged in the back, so he plugged it in the front instead.

Anyway, the sound isn't working in either slot. I've tried the speakers with my laptop and their working fine on that, so that rules out the speakers being the problem.

When I plug the speakers in and out of the back slot, the following application pops up on my desktop, with the message 'what device did you plug in?':



I've provided some screenshots extra below of the sound mixer etc., if there any help.