Hello there. Go to START, SETTINGS, CONTROL PANEL, wait for it to load, LOOK FOR SPEAKER ICON AND CLICK IT. If you have sound controls on your keyboard, You can push the mute button since it is easy to accidentally mute your sound and pushing the mute button again usually toggles between mute and unmute. Hope this helps. Mechbook
another thing is to check to see if your sound card is still being detected by windows. you do that by going into device manger.
for windows xp START-CONTROL PANEL-SYSTEM-HARDWARE TAB-DEVICE MANAGER
WINDOWS VISTA: START-CONTROL PANEL-SYSTEM ICON-click on device manger which is on the left side of the window that appears
Windows 7:START-CONTROL PANNEL-SYSTEM ICON-click on device manger which is on the left side of the window that appears
once in device manger look for something called sound,video, and game controllers. if you do not see anything like that look of icon in there that have exclamation marks or question marks on them and double click on them and see what it says
either way if there are any question or exclamation marks in device manager there is a problem