If you have the sound drivers just install them again and restart your computer.
Let me know if you need more help.
Dont worry now I will go really step by step with images to help you with this process.
1) Reboot the PC
2) While it is rebooting keep tapping the F8 key on your keyboard.
3) You will come to this screen :-
4) You see the option that is highlighted in the image.
Last known good configuration
You need to hit Enter on this one. And wait for it to come back. Once it is back. Check if you have sound back or not ?
Have you changed the cable in the last few days.
Sorry I was away. My system shutdown so had reboot again and get back on net. Sorry for that. Ok so you said that your system was running ok sometime back. Okay. Lets try System Restore and see if that works for you.
Here are the steps for it :-
Click Start, point to All Programs, point to Accessories, point to System Tools, and then click System Restore.
On the Welcome screen, click Restore my computer to an earlier time, and then click Next.
Let me know if this helps. If not I am right here till your issue gets resolved.