Chat feature disappeared

During an all-day training session today (1/31/24) with 25 attendees, 3 students reported that they no longer could find the chat feature, even though they had been using it earlier in the same session. At least one of them was using the browser-based version of GTT and one of the others was using the desktop app version. They tried selecting different screen layout options and tried logging out of the session and then back in again, but the chat was nowhere to be found. The rest of the attendees did not have this issue. In the two years I have been delivering training to class just like this one, I've never run across this problem before. Not sure how to resolve it.