Unable to rotate cab camera

Hi,
I am unable to rotate or view around while in cab. The camera is stuck at its default position. I don't know what caused it as I did not make any change in the files. Need some suggestions, please.
I am unable to rotate or view around while in cab. The camera is stuck at its default position. I don't know what caused it as I did not make any change in the files. Need some suggestions, please.