From your description, there can be a variety of reasons why this happens. I have the following questions to confirm with you:
Does your camera work in other apps?
Does the Teams web client have the same problem?
Do other users within your organization have this issue?
To troubleshoot if it's a camera, go to Settings > device > camera to check that it's working.
If there is a problem, try to fix it by following the steps in the article: My camera isn't working in Teams.
Also, for the questions of Teams:
- When you're on a call or in a meeting, Teams has bandwidth requirements. Follow this article to check whether your bandwidth meets the requirements: Bandwidth requirements.
- Try clearing the cache, clearing your login credentials, and then logging back in.
- Check that the client is up to date.
- If you're unlucky, reinstall the client.
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