A black picture and 'no camera found' are different problems
Pin down the symptom first. If our webcam test says no camera was found, you have a connection or driver problem; if the camera appears in the device list but the preview stays black, the video signal is being blocked somewhere. The two have different fixes, so this distinction is step one.
For the black-picture case there are four usual suspects — a physical shutter, another app holding the camera, blocked permissions, and driver or USB trouble. Checking them in that order, fastest first, resolves most cases within five minutes.
The one-minute fixes: shutter and app conflicts
Start with the embarrassing one, because it's genuinely the most common: the privacy shutter. Many recent laptops and external webcams have a physical slide that covers the lens, and plenty of people forget the sticker or cover they applied months ago. Look straight at the lens and check.
Next, exclusivity. Most webcams can only feed one app at a time — leave Zoom running and every other app gets a black frame. Close every conferencing app, OBS, and camera utility (including ones hiding in the system tray) and retry. If you can't tell what's holding the camera, a reboot is the fastest cure.
Permissions: there are two layers
On Windows, Settings → Privacy → Camera has both a master switch and per-app toggles — check both. On a company laptop, security policy may disable the camera outright; look in Device Manager for a camera marked as disabled. On macOS, grant access per-app under System Settings → Privacy & Security → Camera.
If the camera is black only in the browser, click the camera icon in the address bar to check the site permission, and consider privacy extensions that block the camera API. Our test distinguishes a permission denial from a missing device and says which one you're facing — so the error message itself tells you where to look.
Still black: connection, driver, hardware
For external webcams, change USB ports — skip the hub and plug directly into the PC. A bandwidth-starved port or failing cable produces exactly this symptom: device detected, no video. On laptops, some models disable the camera with an Fn-key combination or dedicated switch; scan your keyboard for a camera icon.
In Windows Device Manager, uninstalling the camera and rebooting reinstalls the driver and cures many stubborn cases. If you've done all of the above and the picture is black in every app and every environment, the camera module itself has likely failed. For a laptop's built-in camera, replacement rarely beats simply buying an external webcam — or using your phone as a webcam, which both Windows (Phone Link) and macOS (Continuity Camera) now support natively.