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The quality of answer you get will depend on the quality of your question and the information you provide.
Did sound work before or not all?
Have you checked the device manager to see whether there are any unusual yellow exclamation marks against your sound drivers?
To do this click on START and right click on my computer and select properties. Once the window opens, click on the hardware tab and click on Device Manager.
Are you sure you have not accidentally engaged mute?
Is there a speaker icon being displayed in your task bar?
☺ When everything else fails, read the instructions.