First, check the webcam outside OBS
Open the webcam test, or on Windows the Camera app. If you see a picture there, the webcam and your computer work, and the fault is in the app that shows nothing: skip to the sections on other apps and on OBS below. If there is no picture there either, start with the connection.
Check the switch, the cable and the port
Some laptops have a physical switch, a button or a sliding shutter that turns the camera off; when it is off, the computer may not see a camera at all. For an external webcam, make sure the cable is firmly in, then try a different USB port on the computer itself rather than a hub.
Close anything else using the camera
If the camera works in one app but not in another, the first app may still be holding it. Close Zoom, Teams, the Camera app, browser tabs with a call open and any other video app, then reopen the one you want. Some capture devices work in only one program at a time; if closing the apps does not release it, restart the computer.
Allow camera access
Windows: go to Settings, Privacy & security, Camera, and turn on Let desktop apps access your camera. OBS and web browsers are desktop apps, so they need this even when the Store apps above it are allowed.
Mac: go to System Settings, Privacy & Security, Camera, and turn access on for OBS, your browser or the app you are using. In Safari, the camera setting for a website is under Safari, Settings, Websites, Camera.
Restart, then check the driver on Windows
Restart the computer: waking it from sleep is not the same as a restart. If the camera is still missing, open Device Manager and choose Action, Scan for hardware changes. Most USB webcams are UVC cameras that work with the driver built into Windows, and Microsoft’s own steps include rolling back the camera driver or switching to that built-in UVC driver. Some antivirus programs and firewalls also block camera access, so check yours.
If it works everywhere except OBS
In OBS, add a Video Capture Device source, open its properties and choose your webcam from the Device list. If the source stays black, the resolution or frame rate set there may be one the camera does not offer; OBS then shows nothing. Set Resolution/FPS Type back to Device Default, or pick a size and rate the camera lists.
If the picture disappears for a moment each time you switch scenes, Deactivate when not showing is on: OBS switches the camera off while its scene is hidden and needs a moment to start it again.
Questions
Why does my webcam work in Zoom but not in OBS? Usually Zoom, or another app, is still holding the camera. Close it completely, then reopen OBS.
Why is my webcam black in OBS when its light is on? Another app may have it, or OBS is asking for a resolution or frame rate the camera does not offer. Set Resolution/FPS Type to Device Default.
My camera works on the webcam test but not in my browser’s video call. Why? Browsers keep a camera permission per website. Allow the camera for that site in the browser’s settings.