Casio CTK 2000- won't work with adapter

"Casio CTK-501 Digital Keyboard Troubleshooting - iFixit" https://www.ifixit.com/Wiki/Casio_CTK-501_Digital_Keyboard_Troubleshooting

I was hoping there might be a Service Manual, but there isn't. Try Casio direct.

https://www.google.com/search?q=casio+ctk+900+service+manual

Lots of Owner's Manuals
..

A Casio Keyboard Service Centre is the best idea.

Casio Ctk-900 Service Manual - CTK900 - HelpOwl.com

https://www.helpowl.com > Casio > CTK900 > Research
Casio Ctk-900 Service Manual - CTK900 ... Get support for Casio CTK900 ... Our system has returned the following pages from the Casio CTK900 data we have on ...

Manual Casio CTK-900 (page 1 of 108) (English) - Libble.eu

https://www.libble.eu > ... > Casio > CTK-900
View and download the Manual of Casio CTK-900 Keyboard (page 1 of 108) (English). ... authorized CASIO Service Provider. AC Adaptor

This could be a loose plug inside the unit or it could be a circuit that is bad. If the plug failed it is from physical damage from something like tripping over it or heavy use. That can be repaired by replacing the plug. If it is something like a component in the unit on a board then it becomes a little more involved.

If you turn it over, you will see the battery compartment. It runs on 6 AA batteries. Here's a link to the manual. Instructions are on page 13.
http://support.casio.com/pdf/008/ctk_496_700_e.pdf

http://www.ebay.com/itm/DC-9V-AC-Adapter-Casio-CTK-4000-CTK-558-CTK700-Keyboard-/221021599357

There probably is NOT a fuse and usual cause is problem with the adapter or the power connectors. Broken solder of the connector in the keyboard is common. Broken cords on the adapter is common as well.

Test the continuity of your Adapter (input and output), if it fails your test replace it with the same output voltage requirement's and watch the polarity as well (negative ground mostly or positive ground).

This could be ay number of things, but assuming that the power supply is fine (the cord isn't broken on either end or down the length of the cord) then it'll probably be a loose connection in the keyboard.
With CTK models if you unscrew the back (not the metal plate) you can open it up, have a look where the connections are going from the power supply socket and re-solder to the board etc if nessesary. Also if you have some strong glue then glueing the adapter in place on the PCB as well as the headphone socket would be a good idea as these connections are used quite a lot and the parts do come loose.

Replace the batteries and get an AC adapter for it! These do NOT work when voltage os too low.