Suddenly no sound!!!

Hi. I'm running Mac OS X 10.6.7

All of a sudden, i go to listen to a song on iTunes, and no sound comes. I click the volume button, and it shows it going up, but there isn't that popping sound.

I'm not using any external speakers, and the sound is up on the menu bar and system preferences. Output says internal speakers.

I have no idea what's wrong!!!!!
Any help would be appreciated.

Thank you.

Oh....

Well, I did it again.

Panicked about a broken computer before restarting it.

Fixed now. Though I still don't know what happened.... Any way to prevent this

I had a similar problem after installing a screen-cast app. It installed some audio driver to record from my iMac mic. I discovered that sound output had been set to this so just switched it back to normal output.