There is a tiny switch inside the headphone jack that turns off the speakers when headphones are connected.
Sometimes this little switch gets stuck on, so that the speakers don't work even when the headphones are disconnected.
Try connecting and disconnecting the headphones several times in quick succession - this can help to release the switch if it is stuck.
Let me know if that works for you..
If that doesn't work, then some electronic testing will be required.
Do you have a multimeter available and some experience in using it, or would you prefer to take the keyboard to a repairer in your area?
Or if you let me know your ZIP code, it may be more convenient for a repairer to visit you?