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Their could be multiple reasons.
1. drivers are corrupt
2. service i stopped
3. hardware problem
you can try following
1. check if windows audio service is running
click on Start->run
this will open a new window showing all the services running on your computer. go to bottom and you will see a service called "Windows Audio". Check the status it should be Running. if it is stopped double click on the service and start it, also set to automatic.
2. if the service is running then uninstall the driver and reinstall it
3. if nothing happens then carry it to the service center and let the manufacturer check it for any hardware issues.