Why my keyboard not turn on?

When I press power button on my Yamaha PSR I455 Keyboard, the LCD comes ON and goes OFF when I release my finger from power button..It's not booting up..i.e.not showing any letters..

1. checked my adapter with multimeter.. it seems ok..

2. checked with batteries..Issue is same.

3. No issue with fuse..

4. No issue with power button

Could anyone please help?