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The following troubleshooting steps below can help to fix a non-working webcam, microphones or speakers/headphones. The steps are more general and not specific to any brand of webcam. Linked below are pages that deal with specific operating systems or hardware type.

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General Solutions

Plug/Unplug (This section doesn't apply to integrated peripherals, e.g. a webcam built into the display.)
1. Unplug the webcam from the computer, then plug it back into the computer. You may also want to try plugging the device into a different USB port than it was plugged into previously.
2. If unplugging and re-plugging does not work, unplug the device again. Restart the computer with the device unplugged. After the computer has restarted and loaded back into the operating system, plug the device into a USB port.
3. If the unplug and re-plug process did not work, continue to the next section.
Restart App and Devices
1. Restart the app trying to access the camera or mic, e.g. Zoom or Collaborate Ultra
2. Restart the computer if you did not already do so in section 1
Miscellaneous Troubleshooting
1. Clear cache and cookies if the camera is being accessed by a web browser (this is much more common in Collaborate than Zoom, since Zoom isn't usually run in a browser.) If you have accidentally hit "deny" when the app asked them for device permission this should solve the problem.
2. Other apps may have control of the device. (Zoom, Collaborate, Skype, etc...) For example close all other apps that could be using the camera, exit and try again.
3. Reboot the computer

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Verify audio input/output by going to System Preferences > Sound > input/output. You will be able to verify that the correct device(s) are recognized and set as the default.
Run software updates, device drivers in macOS are packaged with system updates so this will often resolve issue.
If necessary, download the driver or device software directly from the manufacturer's website. Downloads from third party sites can often be bundled with malware and represent a security risk.