Problem with sound (mic) on webinar

I do remote lecturing about music via Zoom, and have the following setup at home:

webcam for video,

laptop to connect with meeting link and as a monitor,

ipad to play music samples,

Electric piano to play live,

headset (Audio-technica BPHS1) with earphones and attached mic,

All of the above goes through a mixer (Yamaha: MG10XU)

I've used this setup a hundred times very successfully. Yesterday, the mic setup failed me and I don't know why. Symptoms: Either I could hear the host and she/audience couldn't hear me, or she/audience could hear me but I couldn't hear her. I played with the mic settings, and nothing solved the problem. Ultimately, we did the webinar with her (and the audience) hearing me, but I couldn't hear her - which was ok since I was the only one talking. When necessary, we communicated via texts on a phone on the side! It was a very odd experience. 

Does anyone have any thoughts on what the problem might be? (By the way, I tested the system with a friend later in the day, and the same thing happened, so I don't think the problem was on the host's end.)

THANKS.