The "hold down the right-most white key while powering on to reset" is a Yamaha reset scheme. Casio factory resets are menu driven, so if you can not power up the CTK-6200, you can not perform a factory reset ("Initialize") on it. At any rate, the instructions for this are on Page E-100 of the manual. For the most part, Casio keyboards pretty much reset everything, except user data, every time they are powered off and back on.
To be absolutely certain about the AC adaptor, I recommend installing a fresh set of six D cells (Page-E10) and trying to power up with that. If it does, that would indicate a problem with the adaptor, its cord, or the power input jack on the back of the keyboard. If it does not, this would indicate an internal problem that would require opening the case to perform diagnostic tests.