Browser: click the lock icon next to the address and set the camera to "Allow", then refresh the page.
If you have more than one camera, choose the right one from More → Audio settings in the app, or from More in the browser.
Some laptops have a camera switch or cover. Make sure it's open.
My camera works, but students can't see me
Wait a few seconds after turning on the camera, especially if the internet is weak.
If the student sees "Waiting for the teacher's camera…" for a long time, turn your camera off and on again.
If you're teaching from a phone browser, the camera stops when you leave the page and comes back when you return to it. Keep the page open in front of you for the whole lesson.