With the Canon 6D if you see L on the rear panel display screen and find that you are unable to change the Aperture setting in manual mode then don't panic! If you are able to adjust the aperture in Av and the aperture can change in P mode then the lens is almost certainly ok and the issue is a camera setting.
If you put the camera into M, manual mode and when you try to change the aperture the letter L shows in display then the LOCK button on the rear of the camera has become engaged and is preventing you from changing the aperture in Manual mode.
Flick the lock switch off and you will be able to change aperture as normal.
