you might want to check the camera aspect ratio is the same in the AE as it is ion the render globals.., but its not that important.

if you mess up any tool, including cameras.., just go to 'reset.'

if you change your camera settings to HD for example.., then you will need to reflect them in the render globals

if you are using HD.., you will need to change you device ratio and aspect ratio (so the camera settings are the same as the render globals.)