iMac sound is no longer working- help?

This morning i turned on my iMac and the sound is no longer working. When i turn on the computer the start up chime in audible, but after that nothing can be heard through itunes, safari, or anything. When i hit the volume keys on my keyboard it shows that the volume is maxed and underneathe there is a circle with a cross through it. I first tried to resolve the problem by going into system preferernces and disconnecting my keyboard from bluetooth and then reconnecting. That didnt work, so i then put my computer to sleep, then restarted it, then shut it down. Still nothing has changed. I followed this up by resetting the SMC and PRAM, but that also has done nothing. Im not entirely sure what to do! If someone could help me out thatd be great. I feel like there should be a simple solution to this problem, but im just missing it!