1- Check to make sure that you haven't hit the Mute function key by accident. It may be F11 (or close to it)
2- This could the problem with the sound settings or the driver's issue. Try to uninstall the audio driver and then install it again. If you need help with finding the correct drivers, Right click the My Computer icon on the Desktop, select Manage, select Device Manager under Computer Management from the left pane, then expand Sound, video and game controllers from right pane. Right click the Device listed under Sound, video and game controllers, click properties, go to the Details tab, and then select Device Instance Id from the drop down list. Make a note of this Device Instance ID.
If this doesn't fix the issue, try running the diagnostic tests provided by the manufacturer on the sound card.
Do reply with results.