You might have accidentally muted a Youtube video. (If you do that your browser remembers the setting and preserves it for all other Youtube videos until you you cancel the mute).
Try to play a Youtube video and look for the speaker symbol just to the right of the Pause button. Hover your cursor over it. If it says 'Mute' then my suggestion is not the cause of your problem. If it says 'Unmute' then simply clicking on it should fix the problem.
If that doesn't work, try playing a CD or MP3 file on your laptop. If you've got sound the problem is probably with Adobe Flash Player (which Youtube uses to play videos in your browser). If you've got no sound (and you're absolutely sure that you've not got any 'Mute' boxes checked) then you need to reinstall your sound driver.
If it looks like an Adobe Flash Player problem, go to your list of installed programs and remove 'Adobe Flash Player Plug-in'. RESTART YOUR COMPUTER. (That's in capitals because it's IMPORTANT!). Then reinstall Flash Player from here:
http://get.adobe.com/flashplayer/
(NB: Deselect the option to install McAfee Security Scan Plus)
If it looks like you need to reinstall the sound driver, please tell us the exact model number for your laptop, so that we can locate the file on the web for you.