Transmission will not shift passed third gear?

So, my vehicle has been giving me some transmission issues. In order to get the vehicle to move, I have to shift it into 1st and then work my way up the gears like it's a standard, but it's an automatic. An additional problem I've noticed is it won't shift out of third into overdrive. I've asked around and people seem to have a different answer as to why it's doing that. Would my transmission be bad, or would it be something as simple as a sensor?