Yamaha p115 powers on but no buttons or keys work, won't power off

HI. Just took my Yamaha P115 Keyboard out of storage. It was stored upright on its side (maybe a mistake?) for a few months. Now the power comes on (power button light goes on) but no sound comes from any keys. No other buttons seem to work (no led’s come on with instrument select or record buttons for instance).

Also, it won’t power off with the power button (Press-hold normally switches off). I have to unplug to turn it off.

Any suggestions much appreciated.