my imac has no sound

i just came home and decided to watch something online, when i realised i have no sound/ it was working perfectly fine earlier today and everyday before right now. I went to preferences>sound and chose built in speakers for output and the same built in for input and still no sound. i tried to play music in itunes and on youtube and other files on my computer and still nothing. I restarted the computer and still no luck.

HELP!!!!