Casio Privia keyboard PX-100

There are a number of issues that could cause this, the most likely culprit is the power supply, when I say power supply I mean the internal power supply, 
This means that you have already obtained a good "known to work" AC to DC power supply, and tested it on your keyboard to see if that is the problem. 
If you have completed this step, then your next step is to test your unit with batteries only, if the keyboard works, then you know your problem, is internal to the keyboard. 
There are two issues to check, before proceeding please read the following note.
 (a note here for those who have no experience working with internal electronics)
If you have no experience working with internal electronics I would not recommend, opening your keyboard. 
Two reasons, one, will void warranty if it still is under warranty. 
secondly, you could accidently cause more damage to the unit. 
Now if you are still reading, and feeling confident, you know that as a child you have dissembled many many electronic devices, just to see how they work. 
If you do decide to open the box, disconnect the power supply to the unit including any batteries. 
open the box and locate the power on button, clean using a Dielectric cleaning agent, can be obtained at most electronic specialty stores. 
clean the power button, may work but doubtful, usually the switch itself is usually bad, there is a way to test the switch, however, that is beyond the scope of this answer, if you are following this you will most likely have the knowledge to do this. 
The second and more challenging issue will be the internal power supply to the keyboard, or the interface that connects to your ac to dc converter. 
This can be much more trickier, and I wont go into it here. 
If you test your power switch determine that it is good then you should consider getting the unit serviced by a technician. 
If you examine the boards, (electronic circuit boards,) and notice burned, connections, (darker than other circuits) then most likely it will not be worth trying to fix. 
Hope this helps somewhat, if nothing else, take this print out to your local service dealer, and that will help in getting your keyboard repaired, 
Delete this part, before taking this to your dealer, 
Most electronic products can be easily fixed with a power switch, or power supply, however, many dealers will not tell you this and give you an estimate of fixing the unit that far exceeds the actual cost of repair, 
an example, in a local TV repair shop, a friend brought in a wide screen Plasma HD TV, quoted repair, $654.00  problem, 
power supply, actual cost, $75.00 my friend ordered the part online and with the help of a friend installed the new power supply returning his unit to full functionality. 
So beware,