If you are unable to hear any sound, there may be an issue with the sound card, audio drivers, configuration etc. Since you said its happened suddenly, do you remember doing any update etc? There are many steps that we can check but I would advice a quick system restore. Try this:
• Open System Restore by clicking the Start button . In the search box, type System Restore, and then, in the list of results, click System Restore. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
• Follow the steps in the wizard to choose a restore point and restore your computer.
If the above does not help, try the FIXIT option in the link http://windows.microsoft.com/en-US/windows-vista/Tips-for-fixing-common-sound-problems
Here’s to Good Karma