Hey i have this same problem with my old Ap-250. The piano become freezing all of sudden, but the power lights still on. Only the piano mode selection suddenly turned off and Song/demo turned on by itself. After that the mainboard box below started to make some soft noise.
I have to turn it off and wait for some minutes for the piano to work. If i dont wait it wont on.
To turn it off i have to unplug the power. If didnt unplug, after 10like minutes it'll be blasting some loud electricity kinda noise from the speaker.
Hope you get some explanation from Casio and please kindly share it here, im looking towards it. Thank you