Please check these four areas to troubleshoot this issue:
(1) Check to make sure your browser settings are allowing ROOM to access your camera and microphone.
(2) Make sure that you are running ROOM in just one browser tab, not several meetings on several tabs, as multiple instances will adversely affect your performance and even your ability to access a second meeting running concurrently.
(3) Please ensure that you are not running another app in another tab or browser that requires the use of you camera and/or microphone. This results in two (or more) apps competing for the same resources (devices) and either degrading both apps' performances or entirely barring the devices from being used in one of the apps entirely.
(4) Another area to scrutinize are your Device Settings, through which you can change your camera and/or microphone selection. You can access this panel by navigating to the in-meeting's tool tray at the screen's bottom and selecting the More Options (...) icon, then select Device Settings from the popup list as shown below.
The Device Settings panel will look familiar, as everyone invited to a meeting has to first enter an username before entering. In most cases, users leave the default devices displayed, but they can in fact, be displayed and changed at will, any time during the meeting.