go to start and type run, open the run dialog box and type services.msc in the new window scroll down to windows audio. right click and disable, wait a second and right click then enable. this will restart your windows audio service and should bring your audio back with out shutting down. this is a common issue when using HDMI and allowing your screen to go to sleep or turn off. windows detects the calbe is unpluged and believes you will never want to use that audio source again so it defaults to the 3.5mm or if thats not pluged in it just shuts it down entirely.