Are you seeing any error messages in Zoom, such as “Camera not detected” or “Camera is in use by another application”?
Do you have a physical camera privacy shutter or a camera on/off key on your keyboard (often used with the Fn key)?
Also, could you please let me know your computer make and model?
In the meantime, please follow the troubleshooting steps in this Zoom support article: https://support.zoom.com/hc/en/article?id=zm_kb&sysparm_article=KB0068908