Moderator audio disconnect

Long webinar....

we dialed in using telephony, and bridged with web conference.  session went very good. we shared screen, all participant and group in meeting room were happy until we came back from lunch.

Moderator disconnected (hang-up) telephone which was bridged to web conference, but screen (web conference) was still on.

Moderator dial back in, and participant couldn't hear us, but we can hear participant.  we disconnect whole session, re-dial in telephony and web, but still participany couldn't hear us, but we can hear them.

How to troubleshoot? what did we do wrong?  how to prevent this happen again?