i have an iMac 27 inch and recently the volume stopped working completely. It only works in itunes. Help please

While you state that you've tried everything, you don't indicate what that is. Which means you may have tried some of the following.

If iTunes can control the volume, then the sound output hardware would seem to be working.

What (other) software are you using that would output sound? Given that iTunes is working, what's not working?

Have you installed any software that might connect to or use the sound hardware? Anything in particular that was installed around the time that the sound hardware stopped working?

Are you using headphones, or the built-in speakers?

Have you tried adjusting the sound settings using System Preferences > Sound?

If you press and hold the speaker icom in the menu bar, does that control work?