The volume potentiometer most likely needs replacing. The stem that connects the knob to the circuit probably snapped.
I just replaced the one on my PX720, and I was flying blind on what would be a fitting replacement, since there wasn't any documentation. (I did have the service manual, though, but I can't find one for the PX 120) I determined that my volume control had a resistance of 5k ohms, so I got this part and it works fine: https://mou.sr/3IRilM0 . I CANNOT GUARANTEE that this will be the same as the PX120, but who knows.
You might be stuck with disassembling the piano to get to the volume control board, and then soldering a new potentiometer (with the same or similar resistance to the old one) where the old one was. There should be youtube videos of the disassembly process to help you. Alternatively if you can find a place that does circuit board repair, they might be able to offer some help.
In the meantime, you could get some computer speakers with their own volume control and connect them to the headphone jack. This was my solution for a few years.
