Hi Margie,
The most popular cause for such a failure is within the control switch. I can't say this applies to Phoenix Cruisers, but for vehicles in-general. With that understanding, I would start by replacing the control switch.
Just in-case you experience what I do on rare occasion...a senior moment on everyday matters, make sure you click the selection lever twice to the left and twice to the right. The center position disables adjustments.