Host can't be heard in Zoom Webinar, and Chat was disabled

Hi, 

I tested a webinar with a co-host last week, and the sound worked.

Just did the real event and my co-host could not hear me. Using a Mac, I selected computer audio, I tested the audio (hearing) and speaking and it worked.

I had re-started my Mac prior to the zoom webinar

Why could I not be heard?

Why was the Chat disabled, we have never had that before.

Thanks

Emma