Unable to access my camera from a website on my iPhone

Assuming you're using Safari and not some other browser, check Safari's Camera settings:

Settings -> Apps -> Safari -> Camera

and see if it's set to Allow, Ask, or Deny access to the camera.

You need to go to your settings and click on Allow, Ask. Otherwise, try changing your browser. If that doesn't work, the site is not optimized for the iPhone camera and may have a software risk.