Got a used CTK 720 and it has no power. sugestions

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.

You probably have shorted out the power supply. Test it with an ohm meter for proper operation. If it's not working, replace it. If it is working, then the power supply board in the instrument is shorted and you'll need to take it in for repair.

SD or SDHC memory cards must be between 2GB to 32GB

1GB SD card for earlier models or even 2GB SD formatted by earlier models are incompatible (folder structure has changed)

PC drivers available at Casio
http://support.casio.com/en/support/download.php?cid=008&pid=20

If you have a MAC google casio driver for Macs by 3rd party.

The Mac needs the driver, and you may need to select it in Chooser as well. If it is not on the instruction disk, it should be downloadable from the manufacturers website.

Does it have a rubber power on button? If ss, it's probably worn oujt. Replace it.

Thanks Francois. This is good advice. It is posted in the problems and questions section of musical instrument repairs.

Just did it today. Hooked up CTK-720 to Imac with Snow Leopard (10.2.6) using GarageBand.
Steps: 1) Go to : http://code.google.com/p/casiousbmididriver/ 2) Download the CasioUSBMIDIDriver.plugin (see the two choices on the right side of screen) 3) Place the CasioUSBMIDIDriver.Plugin file into the MIDI Driver subfolder: Macintosh HD => Library => Audio => MIDI Drivers 4) Restart 5) Plug in USB cable, open GarageBand, turn on Casio 6) In GraarageBand Open New Project, Start by using Piano, Select Keyboard (command K) in the Window menu 7) You should see the keyboard being played on the screen.
PS: Thank you Francois Ferland.

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

There is a chance that the jack that the power supply plugs into is broken loose or has bad solder joints to the circuit board. I am hoping that you purchased a direct replacement for the power supply.