Well? did you try the jackplug in different holes or not? And did you have the sound turned up when you did so?
Also, click on Start-Control Panel- Sounds and Audio Devices- and click the Volume tab to make sure all levels are "up' and not set to "Mute".
If that doesn't work, stay in Control panel and click on "System" A box labelled "System Properties" will appear. Click on "Hardware" and bugger about in Device Manager to see if your sound devices have yellow exclamation marks against them (which means they aren't working, but at least you'll know they officially have a problem). Sometimes it's just a case of clicking on an icon to solve a problem.
Then check the internet - just type a summary of your situation into Google to see what can be done.
Finally: Was there a time recently when your sound was working? If you go Control Panel- System it offers a System Restore feature which will reset your computer to a happier time when everything was hunky dory.
Unless you've got a Mac, in which case Macs are different.