Lately I have been having trouble adjusting my aperture when my 5 d mark 2 is in manual mode. Usually I am able to turn the thick dial at the back of the camera and adjust the aperture. Lately this is not working. In other modes the dial works fine. Is it my camera? Or have I accidentally changed a setting?
This is a very common issue with the 5D series. You have accidentally locked the rear control-dial. The power-switch next to it has actually 3 positions. When lined up with On, the camera is on but the rear control-dial is disabled. When lined up with the white line which goes to that dial, the camera is on and the dial is enabled.
The other cause for this, apart from the POWER SWITCH setting, is found in the CUSTOM FUNCTIONS SETTINGS of the camera MENU.
Go to "C.Fn III:Autofocus/Drive" and select "-3". You will see three options:
3:Quick Control Dial direct.
Select "0:Normal" and this will free up the Quick Control Dial for aperture setting in manual mode.