Here are some checklists
✔ Software keys to turn on the webcam
In certain computers, there is a function key button that you need to press to turn the webcam on. For example, for Lenovo laptops, it will be "Fn" + "F7" key
✔ Hardware switches to turn on the webcam
Look around your laptop, sometimes there is a toggle switch to turn your webcam on/off
✔ Allowing webcam use in browser
When you entered the classroom, did you "Allow" the use of the camera?
✔ Using the Chrome browser
Are you using the Google Chrome browser? It's the most reliable in getting your video working. If you don't, you can download it here
✔ Windows/Mac settings menu
Is Windows/Mac configured to use the correct microphone?
✔ Installing a proper external webcam driver
If you're using an external webcam like a logitech, you may need to install the driver first.
✔ Plugging the external webcam in, and turning the power on
You can try plugging/unplugging the webcam, and find out of there are an on/off switches
✔ Quitting other software that uses the webcam, such as Skype
Software such as Skype may get in the way of the browser being able to use the webcam.
✔ Refresh your browser page
Sometimes, reloading the page may help enable the webcam to work
✔ Rebooting your computer
Rebookting a computer, especially Windows, sometimes can get a webcam to magically work