My fisheye lens f stop is stuck
You didn't provide the name or model lens, so I can provide generic assistance that applies to many lenses. Hopefully, it will work for you.
Many lenses need to be set to the smallest aperture setting in order to be controlled by the camera. If it is moved off this value, the camera will be unable to change the f-stop value. If it is stuck - meaning the camera can't change it - make sure it is set for the smallest opening (highest number 28, 32, etc.). If it is stuck meaning you are unable to twist the aperture ring on the lens barrel to manually change the setting, look for a small tab that engages the aperture ring. They are often employed to prevent accidental movement of the aperture ring off of the smallest opening.