- the plug is turned on at the mains socket on the wall
- the cable is properly inserted in the back of the keyboard
- the power button is pressed on the keyboard
If none of this works, I suggest trying to plug the keyboard into a different plug socket and repeat the checks above. If this doesn't work, it could be a fault with the power supply (they cost around £ 30 to replace) or the keyboard.
If it transpires that you need to replace the keyboard and you are in the UK, then