That depends on what you did to lose those sounds. Undo that and it should work again.

- It could be a setting. On my Dell netbook, for example <Fn><F5> toggles between sound and no sound. There's also a mute setting in the volume control icon in the notification area and in the audio settings in Control Panel.
- A common Windows way if it isn't a setting: system restore back to when it still worked.
- If system restore doesn't work, system recovery back to factory conditions might be a solution.
- None of the three above will work if it's a hardware issue, however.

Kees