Are you accidentally pushing any buttons on your keyboard? That circle with the / symbol, represents MUTE.
Just saw that you posted a reply in another thread, so I'll recommend the usual...
Mac OS X: Starting up in Safe Mode
Resetting your Mac's PRAM and NVRAM
Resetting the System Management Controller (SMC)
This problem has surfaced from time to time. It seems that Apple has an overly complicated headphone jack in it's iMacs. If you can hear sound through a set of headphones but the speakers don't come back on when you unplug the headphones it is the contact in the headphone jack not making up properly to transfer the sound back to the speakers.
Gently plug and unplug your headphones until the problem resolves itself. Or if it's still under warranty you could take it into the Genius Bar. The problem there might be that the vibration could "fix" it before you get there.