If your screen is frozen during a conference, it could be due to a poor quality Internet connection. You need to restart the conference page.
For Windows OS users: pressing Ctrl+R combo or F5 should do the trick. The page will reload and the process will be restored. If the attempt did not change anything, restart your computer (tablet, smartphone).
If you're using MAC OS, try cmd+R. The page will refresh and the conference access will be restored.
If you're on mobile, swipe down to refresh the page.